fix(element binder): fix memory leak with expando value holding onto node

Expando keys are weak references but the values are not.  The
ElementProbe value holds strong references to the node and the injector
keep it from being garbage collected.

Author: chirayuk

test/core_dom/compiler_spec.dart

@@ -727,6 +727,28 @@ void main() {
             expect(_.rootElement.shadowRoot).toBeNotNull();
           }
         }));
+
+        describe('expando memory', () {
+          Expando expando;
+
+          beforeEach(inject((Expando _expando) => expando = _expando));
+
+          ['shadowy', 'shadowless'].forEach((selector) {
+            it('should release expando when a node is freed ($selector)', async(() {
+              _.rootScope.context['flag'] = true;
+              _.compile('<div><div ng-if=flag><$selector>x</$selector></div></div>');
+              microLeap(); _.rootScope.apply();
+              var element = _.rootElement.querySelector('$selector');
+              if (element.shadowRoot != null) {
+                element = element.shadowRoot;
+              }
+              expect(expando[element]).not.toEqual(null);
+              _.rootScope.context['flag'] = false;
+              microLeap(); _.rootScope.apply();
+              expect(expando[element]).toEqual(null);
+            }));
+          });
+        });
       });
 
       describe('bindings', () {
@@ -849,7 +871,6 @@ void main() {
         expect(_.rootElement.text).toEqual('MyController');
       });
     });
-
   }));
 }
 
@@ -975,6 +996,16 @@ class SimpleComponent {
   }
 }
 
+@Component(
+    selector: 'simple2',
+    template: r'{{name}}(<content>SHADOW-CONTENT</content>)')
+class Simple2Component {
+  Scope scope;
+  Simple2Component(Scope this.scope) {
+    scope.context['name'] = 'INNER';
+  }
+}
+
 @Component(
   selector: 'shadowy',
   template: r'With shadow DOM',