Support dynamic functions for computed annotations.

Alternate take for #3299

src/standard/annotations.html

@@ -145,8 +145,17 @@ TODO(sjmiles): this module should produce either syntactic metadata
           for (var k=0; k<b.parts.length; k++) {
             var p = b.parts[k];
             if (!p.literal) {
-              p.signature = this._parseMethod(p.value);
-              if (!p.signature) {
+              var signature = this._parseMethod(p.value);
+              if (signature) {
+                var method = signature.method;
+                if (this.properties[method]) {
+                  var args = signature.args || [];
+                  args.push(this._parseArg(method));
+                  signature.args = args;
+                  signature.static = false;
+                }
+                p.signature = signature;
+              } else {
                 p.model = this._modelForPath(p.value);
               }
             }

test/unit/bind-elements.html

@@ -582,3 +582,35 @@
   })();
   </script>
 </dom-module>
+
+<dom-module id="x-bind-computed-property">
+  <template>
+    <div id="check">[[translate('Hello World.')]]</div>
+  </template>
+  <script>
+    Polymer({
+      is: 'x-bind-computed-property',
+      properties: {
+        translate: {
+          type: Function,
+          computed: '_computeTranslateFn(translator)'
+        },
+        translator: {
+          type: Function,
+          value: function() {
+            return function(message) {
+              return 'translated: ' + message;
+            }
+          }
+        },
+      },
+
+      _computeTranslateFn: function(translator) {
+        return function(message) {
+          return translator(message);
+        }
+      },
+
+    });
+  </script>
+</dom-module>

test/unit/bind.html

@@ -285,6 +285,36 @@ suite('single-element binding effects', function() {
 
 <script>
 
+suite('computed properties', function() {
+
+  var el;
+
+  setup(function() {
+
+  });
+
+  teardown(function() {
+    document.body.removeChild(el);
+  });
+
+  test('x', function() {
+    el = document.createElement('x-bind-computed-property');
+    document.body.appendChild(el);
+
+    assert.equal(el.$.check.textContent, 'translated: Hello World.');
+
+    el.translator = function(message) {
+      return 'changed: ' + message;
+    }
+
+    assert.equal(el.$.check.textContent, 'changed: Hello World.');
+  });
+
+});
+
+</script>
+
+<script>
 suite('2-way binding effects between elements', function() {
 
   var el;