diff --git a/.eslintrc.json b/.eslintrc.json
index 12b7b508..3f253fb8 100644
--- a/.eslintrc.json
+++ b/.eslintrc.json
@@ -2,7 +2,8 @@
   "extends": "eslint:recommended",
   "ecmaVersion": 6,
   "rules": {
-    "no-console": 0
+    "no-console": 0,
+    "no-var": "error"
   },
   "env": {
     "browser": true,
diff --git a/gulpfile.js b/gulpfile.js
index a6ef501f..b1a79e18 100644
--- a/gulpfile.js
+++ b/gulpfile.js
@@ -219,7 +219,7 @@ gulp.task('release', function(cb) {
 });
 
 gulp.task('lint', function() {
-  return gulp.src(['src/**/*.html', 'test/unit/*.html', 'util/*.js'])
+  return gulp.src(['lib/**/*.html', 'test/unit/*.html', 'util/*.js'])
     .pipe(eslint())
     .pipe(eslint.format())
     .pipe(eslint.failAfterError());
diff --git a/lib/elements/array-selector.html b/lib/elements/array-selector.html
index 62eed7f0..4d9f1df0 100644
--- a/lib/elements/array-selector.html
+++ b/lib/elements/array-selector.html
@@ -377,7 +377,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
    *     ];
    *   },
    *   toggleSelection(e) {
-   *     var item = this.$.employeeList.itemForElement(e.target);
+   *     let item = this.$.employeeList.itemForElement(e.target);
    *     this.$.selector.select(item);
    *   }
    * });
diff --git a/lib/elements/dom-repeat.html b/lib/elements/dom-repeat.html
index 33404f26..a22053a6 100644
--- a/lib/elements/dom-repeat.html
+++ b/lib/elements/dom-repeat.html
@@ -282,7 +282,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
     disconnectedCallback() {
       super.disconnectedCallback();
       this.__isDetached = true;
-      for (var i=0; i<this.__instances.length; i++) {
+      for (let i=0; i<this.__instances.length; i++) {
         this.__detachInstance(i);
       }
     }
@@ -292,8 +292,8 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
       // only perform attachment if the element was previously detached.
       if (this.__isDetached) {
         this.__isDetached = false;
-        var parent = this.parentNode;
-        for (var i=0; i<this.__instances.length; i++) {
+        let parent = this.parentNode;
+        for (let i=0; i<this.__instances.length; i++) {
           this.__attachInstance(i, parent);
         }
       }
@@ -304,7 +304,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
       // until ready, since won't have its template content handed back to
       // it until then
       if (!this.__ctor) {
-        var template = this.template = this.querySelector('template');
+        let template = this.template = this.querySelector('template');
         if (!template) {
           // // Wait until childList changes and template should be there by then
           let observer = new MutationObserver(() => {
@@ -319,7 +319,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
           return false;
         }
         // Template instance props that should be excluded from forwarding
-        var instanceProps = {};
+        let instanceProps = {};
         instanceProps[this.as] = true;
         instanceProps[this.indexAs] = true;
         instanceProps[this.itemsIndexAs] = true;
@@ -328,8 +328,8 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
           parentModel: true,
           instanceProps: instanceProps,
           forwardHostProp: function(prop, value) {
-            var i$ = this.__instances;
-            for (var i=0, inst; (i<i$.length) && (inst=i$[i]); i++) {
+            let i$ = this.__instances;
+            for (let i=0, inst; (i<i$.length) && (inst=i$[i]); i++) {
               inst.forwardHostProp(prop, value);
             }
           },
@@ -358,7 +358,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
     }
 
     __sortChanged(sort) {
-      var methodHost = this.__getMethodHost();
+      let methodHost = this.__getMethodHost();
       this.__sortFn = sort && (typeof sort == 'function' ? sort :
         function() { return methodHost[sort].apply(methodHost, arguments); });
       this.__needFullRefresh = true;
@@ -368,7 +368,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
     }
 
     __filterChanged(filter) {
-      var methodHost = this.__getMethodHost();
+      let methodHost = this.__getMethodHost();
       this.__filterFn = filter && (typeof filter == 'function' ? filter :
         function() { return methodHost[filter].apply(methodHost, arguments); });
       this.__needFullRefresh = true;
@@ -405,8 +405,8 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
       // Simple auto chunkSize throttling algorithm based on feedback loop:
       // measure actual time between frames and scale chunk count by ratio
       // of target/actual frame time
-      var currChunkTime = performance.now();
-      var ratio = this._targetFrameTime / (currChunkTime - this.__lastChunkTime);
+      let currChunkTime = performance.now();
+      let ratio = this._targetFrameTime / (currChunkTime - this.__lastChunkTime);
       this.__chunkCount = Math.round(this.__chunkCount * ratio) || 1;
       this.__limit += this.__chunkCount;
       this.__lastChunkTime = currChunkTime;
@@ -436,8 +436,8 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
     __handleObservedPaths(path) {
       if (this.__observePaths) {
         path = path.substring(path.indexOf('.') + 1);
-        var paths = this.__observePaths;
-        for (var i=0; i<paths.length; i++) {
+        let paths = this.__observePaths;
+        for (let i=0; i<paths.length; i++) {
           if (path.indexOf(paths[i]) === 0) {
             this.__needFullRefresh = true;
             this.__debounceRender(this.__render, this.delay);
@@ -534,21 +534,21 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
     }
 
     __detachInstance(idx) {
-      var inst = this.__instances[idx];
-      for (var i=0; i<inst.children.length; i++) {
-        var el = inst.children[i];
+      let inst = this.__instances[idx];
+      for (let i=0; i<inst.children.length; i++) {
+        let el = inst.children[i];
         inst.root.appendChild(el);
       }
       return inst;
     }
 
     __attachInstance(idx, parent) {
-      var inst = this.__instances[idx];
+      let inst = this.__instances[idx];
       parent.insertBefore(inst.root, this);
     }
 
     __detachAndRemoveInstance(idx) {
-      var inst = this.__detachInstance(idx);
+      let inst = this.__detachInstance(idx);
       if (inst) {
         this.__pool.push(inst);
       }
@@ -556,7 +556,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
     }
 
     __stampInstance(item, instIdx, itemIdx) {
-      var model = {};
+      let model = {};
       model[this.as] = item;
       model[this.indexAs] = instIdx;
       model[this.itemsIndexAs] = itemIdx;
@@ -564,7 +564,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
     }
 
     __insertInstance(item, instIdx, itemIdx) {
-      var inst = this.__pool.pop();
+      let inst = this.__pool.pop();
       if (inst) {
         // TODO(kschaaf): If the pool is shared across turns, hostProps
         // need to be re-set to reused instances in addition to item
@@ -575,8 +575,8 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
       } else {
         inst = this.__stampInstance(item, instIdx, itemIdx);
       }
-      var beforeRow = this.__instances[instIdx + 1];
-      var beforeNode = beforeRow ? beforeRow.children[0] : this;
+      let beforeRow = this.__instances[instIdx + 1];
+      let beforeNode = beforeRow ? beforeRow.children[0] : this;
       this.parentNode.insertBefore(inst.root, beforeNode);
       this.__instances[instIdx] = inst;
       return inst;
@@ -584,7 +584,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
 
     // Implements extension point from Templatize mixin
     _showHideChildren(hidden) {
-      for (var i=0; i<this.__instances.length; i++) {
+      for (let i=0; i<this.__instances.length; i++) {
         this.__instances[i]._showHideChildren(hidden);
       }
     }
@@ -592,17 +592,17 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
     // Called as a side effect of a host items.<key>.<path> path change,
     // responsible for notifying item.<path> changes to inst for key
     __handleItemPath(path, value) {
-      var itemsPath = path.slice(6); // 'items.'.length == 6
-      var dot = itemsPath.indexOf('.');
-      var itemsIdx = dot < 0 ? itemsPath : itemsPath.substring(0, dot);
+      let itemsPath = path.slice(6); // 'items.'.length == 6
+      let dot = itemsPath.indexOf('.');
+      let itemsIdx = dot < 0 ? itemsPath : itemsPath.substring(0, dot);
       // If path was index into array...
       if (itemsIdx == parseInt(itemsIdx, 10)) {
-        var itemSubPath = dot < 0 ? '' : itemsPath.substring(dot+1);
+        let itemSubPath = dot < 0 ? '' : itemsPath.substring(dot+1);
         // See if the item subpath should trigger a full refresh...
         if (!this.__handleObservedPaths(itemSubPath)) {
           // If not, forward to the instance for that index
-          var instIdx = this.__itemsIdxToInstIdx[itemsIdx];
-          var inst = this.__instances[instIdx];
+          let instIdx = this.__itemsIdxToInstIdx[itemsIdx];
+          let inst = this.__instances[instIdx];
           if (inst) {
             let itemPath = this.as + (itemSubPath ? '.' + itemSubPath : '');
             // This is effectively `notifyPath`, but avoids some of the overhead
@@ -628,7 +628,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
      * @return {*} Item associated with the element.
      */
     itemForElement(el) {
-      var instance = this.modelForElement(el);
+      let instance = this.modelForElement(el);
       return instance && instance[this.as];
     }
 
@@ -643,7 +643,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
      *   not correspond to the array index if a user `sort` is applied).
      */
     indexForElement(el) {
-      var instance = this.modelForElement(el);
+      let instance = this.modelForElement(el);
       return instance && instance[this.indexAs];
     }
 
@@ -655,7 +655,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
      *
      * Example:
      *
-     *   var model = modelForElement(el);
+     *   let model = modelForElement(el);
      *   if (model.index < 10) {
      *     model.set('item.checked', true);
      *   }
diff --git a/lib/legacy/legacy-element-mixin.html b/lib/legacy/legacy-element-mixin.html
index 136215bb..e3b9c078 100644
--- a/lib/legacy/legacy-element-mixin.html
+++ b/lib/legacy/legacy-element-mixin.html
@@ -286,7 +286,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
        *   template content.
       */
       instanceTemplate(template) {
-        var dom = /** @type {DocumentFragment} */
+        let dom = /** @type {DocumentFragment} */
           (document.importNode(template._content || template.content, true));
         return dom;
       }
@@ -309,13 +309,13 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
       fire(type, detail, options) {
         options = options || {};
         detail = (detail === null || detail === undefined) ? {} : detail;
-        var event = new Event(type, {
+        let event = new Event(type, {
           bubbles: options.bubbles === undefined ? true : options.bubbles,
           cancelable: Boolean(options.cancelable),
           composed: options.composed === undefined ? true: options.composed
         });
         event.detail = detail;
-        var node = options.node || this;
+        let node = options.node || this;
         node.dispatchEvent(event)
         return event;
       }
@@ -331,14 +331,14 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
        */
       listen(node, eventName, methodName) {
         node = node || this;
-        var hbl = this.__boundListeners ||
+        let hbl = this.__boundListeners ||
           (this.__boundListeners = new WeakMap());
-        var bl = hbl.get(node);
+        let bl = hbl.get(node);
         if (!bl) {
           bl = {};
           hbl.set(node, bl);
         }
-        var key = eventName + methodName;
+        let key = eventName + methodName;
         if (!bl[key]) {
           bl[key] = this._addMethodEventListenerToNode(
             node, eventName, methodName, this);
@@ -357,9 +357,9 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
        */
       unlisten(node, eventName, methodName) {
         node = node || this;
-        var bl = this.__boundListeners && this.__boundListeners.get(node);
-        var key = eventName + methodName;
-        var handler = bl && bl[key];
+        let bl = this.__boundListeners && this.__boundListeners.get(node);
+        let key = eventName + methodName;
+        let handler = bl && bl[key];
         if (handler) {
           this._removeEventListenerFromNode(node, eventName, handler);
           bl[key] = null;
@@ -405,7 +405,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
        * `this.getRootNode().host`.
        */
       get domHost() {
-        var root = this.getRootNode();
+        let root = this.getRootNode();
         return (root instanceof DocumentFragment) ? root.host : root;
       }
 
@@ -459,7 +459,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
        * @return {Array<Node>} List of effctive children.
        */
       getEffectiveChildren() {
-        var list = this.getEffectiveChildNodes();
+        let list = this.getEffectiveChildNodes();
         return list.filter(function(n) {
           return (n.nodeType === Node.ELEMENT_NODE);
         });
@@ -474,9 +474,9 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
        * @return {string} List of effctive children.
        */
       getEffectiveTextContent() {
-        var cn = this.getEffectiveChildNodes();
-        var tc = [];
-        for (var i=0, c; (c = cn[i]); i++) {
+        let cn = this.getEffectiveChildNodes();
+        let tc = [];
+        for (let i=0, c; (c = cn[i]); i++) {
           if (c.nodeType !== Node.COMMENT_NODE) {
             tc.push(c.textContent);
           }
@@ -493,7 +493,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
        * @return {Object<Node>} First effective child node that matches selector.
        */
       queryEffectiveChildren(selector) {
-        var e$ = this.queryDistributedElements(selector);
+        let e$ = this.queryDistributedElements(selector);
         return e$ && e$[0];
       }
 
@@ -521,7 +521,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
        * @return {Array<Node>} List of distributed nodes for the `<slot>`.
        */
       getContentChildNodes(slctr) {
-        var content = this.root.querySelector(slctr || 'slot');
+        let content = this.root.querySelector(slctr || 'slot');
         return content ? Polymer.dom(content).getDistributedNodes() : [];
       }
 
@@ -621,7 +621,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
        */
       isDebouncerActive(jobName) {
         this._debouncers = this._debouncers || {};
-        var debouncer = this._debouncers[jobName];
+        let debouncer = this._debouncers[jobName];
         return !!(debouncer && debouncer.isActive());
       }
 
@@ -633,7 +633,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
        */
       flushDebouncer(jobName) {
         this._debouncers = this._debouncers || {};
-        var debouncer = this._debouncers[jobName];
+        let debouncer = this._debouncers[jobName];
         if (debouncer) {
           debouncer.flush();
         }
@@ -647,7 +647,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
        */
       cancelDebouncer(jobName) {
         this._debouncers = this._debouncers || {}
-        var debouncer = this._debouncers[jobName];
+        let debouncer = this._debouncers[jobName];
         if (debouncer) {
           debouncer.cancel();
         }
@@ -695,12 +695,12 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
        * @return {Element} Newly created and configured element.
        */
       create(tag, props) {
-        var elt = document.createElement(tag);
+        let elt = document.createElement(tag);
         if (props) {
           if (elt.setProperties) {
             elt.setProperties(props);
           } else {
-            for (var n in props) {
+            for (let n in props) {
               elt[n] = props[n];
             }
           }
@@ -834,14 +834,14 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
        * @return {Array} Array containing item removed.
        */
       arrayDelete(arrayOrPath, item) {
-        var index;
+        let index;
         if (Array.isArray(arrayOrPath)) {
           index = arrayOrPath.indexOf(item);
           if (index >= 0) {
             return arrayOrPath.splice(index, 1);
           }
         } else {
-          var arr = Polymer.Path.get(this, arrayOrPath);
+          let arr = Polymer.Path.get(this, arrayOrPath);
           index = arr.indexOf(item);
           if (index >= 0) {
             return this.splice(arrayOrPath, index, 1);
diff --git a/lib/legacy/polymer-fn.html b/lib/legacy/polymer-fn.html
index da95f308..28f45165 100644
--- a/lib/legacy/polymer-fn.html
+++ b/lib/legacy/polymer-fn.html
@@ -31,7 +31,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
     window.Polymer._polymerFn = function(info) {
       // if input is a `class` (aka a function with a prototype), use the prototype
       // remember that the `constructor` will never be called
-      var klass;
+      let klass;
       if (typeof info === 'function') {
         klass = info;
       } else {
diff --git a/lib/mixins/element-mixin.html b/lib/mixins/element-mixin.html
index 9bcb71ec..7224a9fa 100644
--- a/lib/mixins/element-mixin.html
+++ b/lib/mixins/element-mixin.html
@@ -577,7 +577,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
         for (let p in p$) {
           let info = p$[p];
           if (!this._isPropertyPending(p)) {
-            var value = typeof info.value == 'function' ?
+            let value = typeof info.value == 'function' ?
               info.value.call(this) :
               info.value;
             if (this._hasPropertyEffect(p)) {
diff --git a/lib/mixins/property-accessors.html b/lib/mixins/property-accessors.html
index dcb5cd5b..9bef8044 100644
--- a/lib/mixins/property-accessors.html
+++ b/lib/mixins/property-accessors.html
@@ -18,9 +18,9 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
 
   'use strict';
 
-  var caseMap = Polymer.CaseMap;
+  let caseMap = Polymer.CaseMap;
 
-  var microtask = Polymer.Async.microTask;
+  let microtask = Polymer.Async.microTask;
 
   // Save map of native properties; this forms a blacklist or properties
   // that won't have their values "saved" by `saveAccessorValue`, since
@@ -186,7 +186,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
       _attributeToProperty(attribute, value, type) {
         // Don't deserialize back to property if currently reflecting
         if (!this.__serializing) {
-          var property = caseMap.dashToCamelCase(attribute);
+          let property = caseMap.dashToCamelCase(attribute);
           this[property] = this._deserializeValue(value, type);
         }
       }
@@ -221,7 +221,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
        * @param {string} attribute Attribute name to serialize to.
        */
       _valueToNodeAttribute(node, value, attribute) {
-        var str = this._serializeValue(value);
+        let str = this._serializeValue(value);
         if (str === undefined) {
           node.removeAttribute(attribute);
         } else {
diff --git a/lib/mixins/property-effects.html b/lib/mixins/property-effects.html
index d27b67e5..894c8afe 100644
--- a/lib/mixins/property-effects.html
+++ b/lib/mixins/property-effects.html
@@ -410,8 +410,8 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
    * @private
    */
   function runComputedEffect(inst, property, props, oldProps, info) {
-    var result = runMethodEffect(inst, property, props, oldProps, info);
-    var computedProp = info.methodInfo;
+    let result = runMethodEffect(inst, property, props, oldProps, info);
+    let computedProp = info.methodInfo;
     if (inst.__propertyEffects && inst.__propertyEffects[computedProp]) {
       inst._setPendingProperty(computedProp, result, true);
     } else {
diff --git a/lib/utils/path.html b/lib/utils/path.html
index b0e839a5..8e6a3e29 100644
--- a/lib/utils/path.html
+++ b/lib/utils/path.html
@@ -55,7 +55,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
      * @return {string} Root property name
      */
     root: function(path) {
-      var dotIndex = path.indexOf('.');
+      let dotIndex = path.indexOf('.');
       if (dotIndex === -1) {
         return path;
       }
@@ -146,10 +146,10 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
      */
     normalize: function(path) {
       if (Array.isArray(path)) {
-        var parts = [];
-        for (var i=0; i<path.length; i++) {
-          var args = path[i].toString().split('.');
-          for (var j=0; j<args.length; j++) {
+        let parts = [];
+        for (let i=0; i<path.length; i++) {
+          let args = path[i].toString().split('.');
+          for (let j=0; j<args.length; j++) {
             parts.push(args[j]);
           }
         }
@@ -194,14 +194,14 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
      *  fully dereferenced.
      */
     get: function(root, path, info) {
-      var prop = root;
-      var parts = this.split(path);
+      let prop = root;
+      let parts = this.split(path);
       // Loop over path parts[0..n-1] and dereference
-      for (var i=0; i<parts.length; i++) {
+      for (let i=0; i<parts.length; i++) {
         if (!prop) {
           return;
         }
-        var part = parts[i];
+        let part = parts[i];
         prop = prop[part];
       }
       if (info) {
@@ -221,13 +221,13 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
      * @return {string | undefined} The normalized version of the input path
      */
     set: function(root, path, value) {
-      var prop = root;
-      var parts = this.split(path);
-      var last = parts[parts.length-1];
+      let prop = root;
+      let parts = this.split(path);
+      let last = parts[parts.length-1];
       if (parts.length > 1) {
         // Loop over path parts[0..n-2] and dereference
-        for (var i=0; i<parts.length-1; i++) {
-          var part = parts[i];
+        for (let i=0; i<parts.length-1; i++) {
+          let part = parts[i];
           prop = prop[part];
           if (!prop) {
             return;
diff --git a/test/.eslintrc.json b/test/.eslintrc.json
index f6ee2a64..2e60b170 100644
--- a/test/.eslintrc.json
+++ b/test/.eslintrc.json
@@ -2,6 +2,9 @@
   "env": {
     "mocha": true
   },
+  "rules": {
+    "no-var": "off"
+  },
   "globals": {
     "assert": true,
     "sinon": true,