dart2js: --fast-startup: updateHolders always copies properties

pkg/compiler/lib/src/js_emitter/startup_emitter/fragment_emitter.dart

Index: pkg/compiler/lib/src/js_emitter/startup_emitter/fragment_emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/startup_emitter/fragment_emitter.dart b/pkg/compiler/lib/src/js_emitter/startup_emitter/fragment_emitter.dart
index 5722d129365294500d9c229b4912f46411c6628e..a193b1e904f5fd69364c91d96fe421779f96507b 100644
--- a/pkg/compiler/lib/src/js_emitter/startup_emitter/fragment_emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/startup_emitter/fragment_emitter.dart
@@ -60,7 +60,8 @@ const String mainBoilerplate = '''
 function copyProperties(from, to) {
   var keys = Object.keys(from);
   for (var i = 0; i < keys.length; i++) {
-    to[keys[i]] = from[keys[i]];
+    var key = keys[i];
+    to[key] = from[key];
   }
 }
 
@@ -275,15 +276,7 @@ function updateTypes(newTypes) {
 // Updates the given holder with the properties of the [newHolder].
 // This function is used when a deferred fragment is initialized.
 function updateHolder(holder, newHolder) {
-  // Firefox doesn't like when important objects have their prototype chain
-  // updated. We therefore do this only on V8.
-  if (isChrome) {
-    var oldPrototype = holder.__proto__;
-    newHolder.__proto__ = oldPrototype;
-    holder.__proto__ = newHolder;
-  } else {
-    copyProperties(newHolder, holder);
-  }
+  copyProperties(newHolder, holder);
   return holder;
 }