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

lib/runtime/dart_sdk.js

Index: lib/runtime/dart_sdk.js
diff --git a/lib/runtime/dart_sdk.js b/lib/runtime/dart_sdk.js
index d040be4d3579775dc805a9510efb365f5f16a923..7455960aa5c512b8ecbe6fb65933a694be006610 100644
--- a/lib/runtime/dart_sdk.js
+++ b/lib/runtime/dart_sdk.js
@@ -200,8 +200,22 @@ dart_library.library('dart_sdk', null, /* Imports */[
   dart.defineExtensionNames = function(names) {
     return names.forEach(dart.getExtensionSymbol);
   };
+  dart.tick1 = function() {
+    dart._t1 = dart._t1 + 1;
+    if (dart._t1 % 100 == 0) console.log([dart._t1, dart._t2]);
+  };
+  dart.tick2 = function() {
+    dart._t2 = dart._t2 + 1;
+    if (dart._t2 % 100 == 0) console.log([dart._t1, dart._t2]);
+  };
   dart._installProperties = function(jsProto, extProto) {
+    dart.tick1();
     let coreObjProto = core.Object.prototype;
+    dart._installProperties2(jsProto, extProto, coreObjProto);
+    jsProto._xxx = extProto;
+  };
+  dart._installProperties2 = function(jsProto, extProto, coreObjProto) {
+    dart.tick2();
     if (extProto === coreObjProto) {
       let names = dart.getOwnPropertyNames(coreObjProto);
       for (let i = 0; i < names.length; ++i) {
@@ -212,7 +226,10 @@ dart_library.library('dart_sdk', null, /* Imports */[
       return;
     }
     if (jsProto !== extProto) {
-      dart._installProperties(jsProto, extProto.__proto__);
+      let extParent = extProto.__proto__;
+      if (jsProto.__proto__._xxx !== extParent) {
+        dart._installProperties2(jsProto, extParent, coreObjProto);
+      }
     }
     dart.copyTheseProperties(jsProto, extProto, dart.getOwnPropertySymbols(extProto));
   };
@@ -998,6 +1015,8 @@ dart_library.library('dart_sdk', null, /* Imports */[
   dart._staticSig = Symbol("sigStatic");
   dart._extensionType = Symbol("extensionType");
   dart.dartx = dartx;
+  dart._t1 = 0;
+  dart._t2 = 0;
   dart._jsIterator = Symbol("_jsIterator");
   dart._current = Symbol("_current");
   dart._AsyncStarStreamController = class _AsyncStarStreamController {