Add accessor property to properties object

This forces a property listed in `properties` to create a property accessor regardless of whether or not it is needed for effects. This allows using `_propertiesChanged` to process changes for properties that may not have effects.

lib/mixins/element-mixin.html

@@ -352,6 +352,10 @@
      * * observer: {string} name of a method that runs when the property
      * changes. The arguments of the method are (value, previousValue).
      *
+     * * accessor: {boolean} Set to true to force a property accessor to be
+     * created regardless of whether or not one is needed for other property
+     * effects.
+     *
      * Note: Users may want control over modifying property
      * effects via subclassing. For example, a user might want to make a
      * reflectToAttribute property not do so in a subclass. We've chosen to
@@ -397,6 +401,10 @@
       if (info.observer) {
         proto._createPropertyObserver(name, info.observer, allProps[info.observer]);
       }
+      // add an accessor if `accessor` is true
+      if (info.accessor) {
+        proto._createPropertyAccessor(name);
+      }
     }
 
     /**

test/unit/polymer.element.html

@@ -38,6 +38,9 @@
             },
             computedProp: {
               computed: '_compute(computedPropDep)'
+            },
+            accessor: {
+              accessor: true
             }
           };
         }
@@ -383,6 +386,13 @@ <h1>Sub template</h1>
       assert.equal(el.computedProp, true);
     });
 
+    test('properties accessor: true creates accessor', function() {
+      assert.isTrue(el.__dataHasAccessor.accessor);
+      el._propertiesChanged = sinon.spy();
+      el.accessor = 'hi';
+      assert.equal(el._propertiesChanged.args[0][0].accessor, 'hi');
+    });
+
     test('attributes', function() {
       var fixtureEl = fixture('my-element-attr');
       assert.equal(fixtureEl.prop, 'attr');