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

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

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart2js.js_emitter.startup_emitter.model_emitter;
 
 /// The name of the property that stores the tear-off getter on a static
 /// function.
 ///
 /// This property is only used when isolates are used.
@@ skipping 42 matching lines @@
 // we can use "global" names however we want. As long as we don't shadow
 // JavaScript variables (like `Array`) we are free to chose whatever variable
 // names we want. Furthermore, the pretty-printer minifies local variables, thus
 // reducing their size.
 const String mainBoilerplate = '''
 (function dartProgram() {
 // Copies the own properties from [from] to [to].
 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];
   }
 }
 
 // Only use direct proto access to construct the prototype chain (instead of
 // copying properties) on platforms where we know it works well (Chrome / d8).
 var supportsDirectProtoAccess = #directAccessTestExpression;
 
 var functionsHaveName = (function() {
   function t() {};
   return (typeof t.name == 'string')
 })();
 
 var isChrome = (typeof window != 'undefined') &&

[Siggi Cherem (dart-lang), 2017/05/17 17:57:40]

I believe this was only used for updateHolders, maybe we can drop this
definition too?

[sra1, 2017/05/18 00:33:53]

Done.

     (typeof window.chrome != 'undefined');
 
 // Sets the name property of functions, if the JS engine doesn't set the name
 // itself.
 // As of 2015 only IE doesn't set the name.
 function setFunctionNamesIfNecessary(holders) {
   if (functionsHaveName) return;
   for (var i = 0; i < holders.length; i++) {
     var holder = holders[i];
     var keys = Object.keys(holder);
@@ skipping 181 matching lines @@
   // This relies on the fact that types are added *after* the tear-offs have
   // been installed. The tear-off function uses the types-length to figure
   // out at which offset its types are located. If the types were added earlier
   // the offset would be wrong.
   types.push.apply(types, 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;
 }
 
 // Every deferred hunk (i.e. fragment) is a function that we can invoke to
 // initialize it. At this moment it contributes its data to the main hunk.
 function initializeDeferredHunk(hunk) {
   // Update the typesOffset for the next deferred library.
   typesOffset = #embeddedTypes.length;
 
   // TODO(floitsch): extend natives.
@@ skipping 1266 matching lines @@
     }
     statements.add(js.js.statement("setOrUpdateInterceptorsByTag(#);",
         js.objectLiteral(interceptorsByTag)));
     statements.add(
         js.js.statement("setOrUpdateLeafTags(#);", js.objectLiteral(leafTags)));
     statements.addAll(subclassAssignments);
 
     return wrapPhase('nativeSupport', statements);
   }
 }