Feature-detect browser for assignment to __proto__.

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 46 matching lines @@
 const String mainBoilerplate = '''
 (function() {
 // 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 supportsDirectProtoAccess = (function () {
-  var cls = function () {};
-  cls.prototype = {'p': {}};
-  var object = new cls();
-  return object.__proto__ &&
-         object.__proto__.p === cls.prototype.p;
-})();
+// 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') &&
     (typeof window.chrome != 'undefined');
 
 // Sets the name property of functions, if the JS engine doesn't set the name
@@ skipping 263 matching lines @@
 
 // Sets up the native support.
 // Native-support uses setOrUpdateInterceptorsByTag and setOrUpdateLeafTags.
 #nativeSupport;
 
 // Invokes main (making sure that it records the 'current-script' value).
 #invokeMain;
 })()
 ''';
 
+/// An expression that returns `true` if `__proto__` can be assigned to stitch
+/// together a prototype chain, and the performance is good.
+const String directAccessTestExpression = r'''
+  (function () {
+    var cls = function () {};
+    cls.prototype = {'p': {}};
+    var object = new cls();
+    if (!(object.__proto__ && object.__proto__.p === cls.prototype.p))
+      return false;
+    
+    try {
+      // Are we running on a platform where the performance is good?
+      // (i.e. Chrome or d8).
+
+      // Chrome userAgent?
+      if (typeof navigator != "undefined" &&
+          typeof navigator.userAgent == "string" &&
+          navigator.userAgent.indexOf("Chrome/") >= 0) return true;
+      // d8 version() looks like "N.N.N.N", jsshell version() like "N".
+      if (typeof version == "function" &&
+          version.length == 0) {
+        var v = version();
+        if (/^\d+\.\d+\.\d+\.\d+$/.test(v)) return true;
+      }
+    } catch(_) {}
+
+    return false;
+  })()
+''';
+
 /// Deferred fragments (aka 'hunks') are built similarly to the main fragment.
 ///
 /// However, at specific moments they need to contribute their data.
 /// For example, once the holders have been created, they are included into
 /// the main holders.
 const String deferredBoilerplate = '''
 function(inherit, mixin, lazy, makeConstList, convertToFastObject,
          installTearOff, setFunctionNamesIfNecessary, updateHolder, updateTypes,
          setOrUpdateInterceptorsByTag, setOrUpdateLeafTags,
          #embeddedGlobalsObject, holdersList, #staticState) {
@@ skipping 62 matching lines @@
   }
 
   js.Statement emitMainFragment(Program program,
       Map<DeferredFragment, _DeferredFragmentHash> deferredLoadHashes) {
     MainFragment fragment = program.fragments.first;
 
     Iterable<Holder> nonStaticStateHolders =
         program.holders.where((Holder holder) => !holder.isStaticStateHolder);
 
     return js.js.statement(mainBoilerplate, {
+      'directAccessTestExpression': js.js(directAccessTestExpression),
       'typeNameProperty': js.string(ModelEmitter.typeNameProperty),
       'cyclicThrow': backend.emitter
           .staticFunctionAccess(backend.helpers.cyclicThrowHelper),
       'operatorIsPrefix': js.string(namer.operatorIsPrefix),
       'tearOffCode': new js.Block(buildTearOffCode(backend)),
       'embeddedTypes': generateEmbeddedGlobalAccess(TYPES),
       'embeddedInterceptorTags':
           generateEmbeddedGlobalAccess(INTERCEPTORS_BY_TAG),
       'embeddedLeafTags': generateEmbeddedGlobalAccess(LEAF_TAGS),
       'embeddedGlobalsObject': js.js("init"),
@@ skipping 865 matching lines @@
     }
     statements.add(js.js.statement("setOrUpdateInterceptorsByTag(#);",
         js.objectLiteral(interceptorsByTag)));
     statements.add(
         js.js.statement("setOrUpdateLeafTags(#);", js.objectLiteral(leafTags)));
     statements.add(subclassAssignment);
 
     return new js.Block(statements);
   }
 }