Optimize length access on all JSIndexable things.

tests/compiler/dart2js/mock_compiler.dart

 // Copyright (c) 2012, 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 mock_compiler;
 
 import "package:expect/expect.dart";
 import 'dart:collection';
 import 'dart:uri';
 
@@ skipping 59 matching lines @@
   dynamic JS(String typeDescription, String codeTemplate,
     [var arg0, var arg1, var arg2, var arg3, var arg4, var arg5, var arg6,
      var arg7, var arg8, var arg9, var arg10, var arg11]) {}''';
 
 const String DEFAULT_INTERCEPTORSLIB = r'''
   class Interceptor {
     toString() {}
     bool operator==(other) => identical(this, other);
     noSuchMethod(im) { throw im; }
   }
-  abstract class JSIndexable {}
+  abstract class JSIndexable {
+    get length;
+  }
   class JSArray extends Interceptor implements List, JSIndexable {
     var length;
     operator[](index) {}
     operator[]=(index, value) {}
     var add;
   }
   class JSMutableArray extends JSArray {}
   class JSFixedArray extends JSMutableArray {}
   class JSExtendableArray extends JSMutableArray {}
   class JSString extends Interceptor implements String, JSIndexable {
@@ skipping 306 matching lines @@
   }
 }
 
 class MockDeferredLoadTask extends DeferredLoadTask {
   MockDeferredLoadTask(Compiler compiler) : super(compiler);
 
   void registerMainApp(LibraryElement mainApp) {
     // Do nothing.
   }
 }