Use _extensionType for detecting prototype inheritance.

tool/input_sdk/private/ddc_runtime/classes.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.
 
 /// This library defines the operations that define and manipulate Dart
 /// classes.  Included in this are:
 ///   - Generics
 ///   - Class metadata
 ///   - Extension methods
 ///
@@ skipping 262 matching lines @@
     sym = JS('', 'Symbol("dartx." + #.toString())', name);
     JS('', 'dartx[#] = #', name, sym);
   }
   return sym;
 }
 
 defineExtensionNames(names) =>
     JS('', '#.forEach(#)', names, getExtensionSymbol);
 
 
-/// A map from peer class prototypes to the Dart class prototype.  This is used
-/// to recognize when Dart subclass inheritance corresponds to JavaScript
-/// prototype inheritance.
-final _installedDartPeers = JS('', 'new Map()');
-
 /// Install properties in prototype-first order.  Properties / descriptors from
 /// more specific types should overwrite ones from less specific types.
 void _installProperties(jsProto, extProto) {
 
   // This relies on the Dart type literal evaluating to the JavaScript
   // constructor.
   var coreObjProto = JS('', '#.prototype', Object);
 
+  var parentsExtension =
+      JS('', '(#.__proto__)[#]', jsProto, _extensionType);
   var installedParent =
-      JS('', '#.get(#.__proto__)', _installedDartPeers, jsProto);
+      JS('', '# && #.prototype', parentsExtension, parentsExtension);
 
   _installProperties2(jsProto, extProto, coreObjProto, installedParent);
-
-  // Mark this jsProto as being the prototype for the extension class.
-  JS('', '#.set(#, #)', _installedDartPeers, jsProto, extProto);
 }
 
 void _installProperties2(jsProto, extProto, coreObjProto, installedParent) {
   if (JS('bool', '# === #', extProto, coreObjProto)) {
     _installPropertiesForObject(jsProto, coreObjProto);
     return;
   }
   if (JS('bool', '# !== #', jsProto, extProto)) {
     var extParent = JS('', '#.__proto__', extProto);
 
@@ skipping 108 matching lines @@
 })()''');
 
 /// Sets the element type of a list literal.
 list(obj, elementType) =>
     JS('', '$setType($obj, ${getGenericClass(JSArray)}($elementType))');
 
 setBaseClass(derived, base) => JS('', '''(() => {
   // Link the extension to the type it's extending as a base class.
   $derived.prototype.__proto__ = $base.prototype;
 })()''');