Feature-detect browser for assignment to __proto__.

pkg/compiler/lib/src/js_emitter/full_emitter/emitter.dart

 // Copyright (c) 2014, 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.
 
 library dart2js.js_emitter.full_emitter;
 
 import 'dart:collection' show HashMap;
 import 'dart:convert';
 
 import 'package:js_runtime/shared/embedded_names.dart' as embeddedNames;
@@ skipping 1049 matching lines @@
     if (compiler.options.hasIncrementalSupport) {
       supportsDirectProtoAccess = js.statement(r'''
         var supportsDirectProtoAccess = false;
       ''');
     } else {
       supportsDirectProtoAccess = js.statement(r'''
         var supportsDirectProtoAccess = (function () {
           var cls = function () {};
           cls.prototype = {'p': {}};
           var object = new cls();
-          return object.__proto__ &&
-                 object.__proto__.p === cls.prototype.p;
-         })();
+          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;
+        })();
       ''');
     }
 
     return supportsDirectProtoAccess;
   }
 
   jsAst.Expression generateLibraryDescriptor(
       LibraryElement library, Fragment fragment) {
     var uri = "";
     if (!compiler.options.enableMinification || backend.mustPreserveUris) {
@@ skipping 1048 matching lines @@
     if (cachedElements.isEmpty) return;
     for (Element element in compiler.enqueuer.codegen.newlyEnqueuedElements) {
       if (element.isInstanceMember) {
         cachedClassBuilders.remove(element.enclosingClass);
 
         nativeEmitter.cachedBuilders.remove(element.enclosingClass);
       }
     }
   }
 }