Make renderedItemCount readOnly & add tests.

src/lib/template/dom-repeat.html

@@ -199,7 +199,8 @@ Then the `observe` property should be configured as follows:
        */
       renderedItemCount: {
         type: Number,
-        notify: true
+        notify: true,
+        readOnly: true
       },
 
       /**
@@ -450,7 +451,7 @@ Then the `observe` property should be configured as follows:
       // the same item.
       this._pool.length = 0;
       // Set rendered item count 
-      this.renderedItemCount = this._instances.length;
+      this._setRenderedItemCount(this._instances.length);
       // Notify users
       this.fire('dom-change');
       // Check to see if we need to render more items

test/unit/dom-repeat.html

@@ -3311,6 +3311,23 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
         assert.equal(repeater3.keyForElement(stamped1[4]), coll3.getKey(items3[2]));
       });
 
+      test('renderedItemCount', function() {
+         var repeater1 = primitive.$.repeater1;
+         primitive.items = [ 'a', 'b', 'c', 'd', 'e' ];
+         repeater1.render();
+         assert.equal(repeater1.renderedItemCount, 5, 'renderedItemCount is incorrect');
+         repeater1.renderedItemCount = 0;
+         assert.equal(repeater1.renderedItemCount, 5, 'renderedItemCount is writable');
+         repeater1.filter = function(item) {
+           return (item != 'a' && item != 'e');
+         }
+         repeater1.render();
+         assert.equal(repeater1.renderedItemCount, 3, 'renderedItemCount incorrect after filter');
+         // reset repeater
+         repeater1.filter = undefined;
+         repeater1.render();
+      });
+
       test('__hideTemplateChildren__', function() {
         // Initially all showing
         var stamped1 = Polymer.dom(primitive.$.container1).querySelectorAll('*:not(template)');