Ensure that dart:js does not attempt to load cached proxies from prototypes.

sdk/lib/js/dart2js/js_dart2js.dart

Index: sdk/lib/js/dart2js/js_dart2js.dart
diff --git a/sdk/lib/js/dart2js/js_dart2js.dart b/sdk/lib/js/dart2js/js_dart2js.dart
index b1003d98d1490c92fb6b38e04a45e293a1aa4bad..576cb23451a596c23f569eecca11f7185d2cebbb 100644
--- a/sdk/lib/js/dart2js/js_dart2js.dart
+++ b/sdk/lib/js/dart2js/js_dart2js.dart
@@ -471,6 +471,13 @@ bool _defineProperty(o, String name, value) {
   return false;
 }
 
+Object _getOwnProperty(o, String name) {
+  if (JS('bool', 'Object.prototype.hasOwnProperty.call(#, #)', o, name)) {
+    return JS('', '#[#]', o, name);
+  }
+  return null;
+}
+
 bool _isLocalObject(o) => JS('bool', '# instanceof Object', o);
 
 dynamic _convertToJS(dynamic o) {
@@ -498,7 +505,7 @@ dynamic _convertToJS(dynamic o) {
 }
 
 Object _getJsProxy(o, String propertyName, createProxy(o)) {
-  var jsProxy = JS('', '#[#]', o, propertyName);
+  var jsProxy = _getOwnProperty(o, propertyName);
   if (jsProxy == null) {
     jsProxy = createProxy(o);
     _defineProperty(o, propertyName, jsProxy);
@@ -543,7 +550,7 @@ JsObject _wrapToDart(o) {
 }
 
 Object _getDartProxy(o, String propertyName, createProxy(o)) {
-  var dartProxy = JS('', '#[#]', o, propertyName);
+  var dartProxy = _getOwnProperty(o, propertyName);
   // Temporary fix for dartbug.com/15193
   // In some cases it's possible to see a JavaScript object that
   // came from a different context and was previously proxied to