Get Polymer compiling clean under closure recommended flags

With these changes we have zero errors and zero warnings with `RECOMMENDED_FLAGS`!

Most of the changes were adding `@override` for methods and properties in mixins. Apparently if you implement an interface you need to say `@override` for each method or property on the interface. This combines with our mixin strategy to the tune of needing to add `@override` on every non-private method and property.

I'm not sure this is intended behavior of the compiler. Filed https://github.com/google/closure-compiler/issues/3137 to see if it is.

externs/polymer-externs.js

@@ -48,6 +48,7 @@ PolymerInit.prototype.hostAttributes;
 /** @type {(!Object<string, string> | undefined)} */
 PolymerInit.prototype.listeners;
 
+/** @record */
 let PolymerElementConstructor = function () {};
 /** @type {(string | undefined)} */
 PolymerElementConstructor.is;
@@ -78,6 +79,26 @@ function Polymer(init){}
  */
 Polymer.sanitizeDOMValue;
 
+/**
+ * @type {boolean}
+ */
+Polymer.passiveTouchGestures;
+
+/**
+ * @type {boolean}
+ */
+Polymer.strictTemplatePolicy;
+
+/**
+ * @type {boolean}
+ */
+Polymer.allowTemplateFromDomModule;
+
+/**
+ * @type {string}
+ */
+Polymer.rootPath;
+
 /**
  * @param {string} string
  * @param {Object} obj
@@ -174,7 +195,7 @@ var PolymerDeepPropertyChange;
  * @constructor
  * @template T
  */
-let DomRepeatEvent = function() {};
+var DomRepeatEvent = function() {};
 
 /**
  * @type {{