Dont install methods if implementation superclass corresponds to an installed extension class

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 261 matching lines @@
   if (sym == null) {
     sym = JS('', 'Symbol("dartx." + #.toString())', name);
     JS('', 'dartx[#] = #', name, sym);
   }
   return sym;
 }
 
 defineExtensionNames(names) =>
     JS('', '#.forEach(#)', names, getExtensionSymbol);
 
+
+int _t1 = 0, _t2 = 0;
+void tick1() {
+  _t1 = JS('int', '# + 1', _t1);
+  if (JS('bool', '# % 100 == 0', _t1)) JS('', 'console.log([#, #])', _t1, _t2);
+}
+void tick2() {
+  _t2 = JS('int', '# + 1', _t2);
+  if (JS('bool', '# % 100 == 0', _t2)) JS('', 'console.log([#, #])', _t1, _t2);
+}
+
 // Install properties in prototype order.  Properties / descriptors from
 // more specific types should overwrite ones from less specific types.
 _installProperties(jsProto, extProto) {
+  tick1();
   var coreObjProto = JS('', '#.prototype', Object);
+  _installProperties2(jsProto, extProto, coreObjProto);
+  // Mark this jsProto as being the prototype for the extension class.
+  // TODO(sra): Fix the tagging.
+  JS('', '#._xxx = #', jsProto, extProto);
+}
+
+_installProperties2(jsProto, extProto, coreObjProto) {
+  tick2();
   if (JS('bool', '# === #', extProto, coreObjProto)) {
      // core.Object members need to be copied from the non-symbol name to the
      // symbol name.
     var names = getOwnPropertyNames(coreObjProto);
     for (int i = 0; i < JS('int', '#.length', names); ++i) {
       var name = JS('', '#[#]', names, i);
       var desc = getOwnPropertyDescriptor(coreObjProto, name);
       defineProperty(jsProto, getExtensionSymbol(name), desc);
     }
     return;
   }
   if (JS('bool', '# !== #', jsProto, extProto)) {
-    _installProperties(jsProto, JS('', '#.__proto__', extProto));
+    var extParent = JS('', '#.__proto__', extProto);
+    // If the js parent is the extension class parent, we inherit all the
+    // extension class methods via prototype inheritance.
+    if(JS('bool', '#.__proto__._xxx !== #', jsProto, extParent)) {

[Leaf, 2016/05/16 18:03:07]

Maybe use a symbol to be more robust?  Or maybe just a better obfuscated name
(e.g. __ddc_extension_prototype or something)?

+      _installProperties2(jsProto, extParent, coreObjProto);
+    }
   }
   copyTheseProperties(jsProto, extProto, getOwnPropertySymbols(extProto));
 }
 ///
 /// Copy symbols from the prototype of the source to destination.
 /// These are the only properties safe to copy onto an existing public
 /// JavaScript class.
 ///
 registerExtension(jsType, dartExtType) => JS('', '''(() => {
   // TODO(vsm): Not all registered js types are real.
@@ skipping 78 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;
 })()''');