Optimize length access on all JSIndexable things.

tests/utils/dummy_compiler_test.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.
 
 // Smoke test of the dart2js compiler API.
 library dummy_compiler;
 
 import 'dart:async';
 import 'dart:uri';
 
@@ skipping 25 matching lines @@
                   class LinkedHashMap {}
                   identical(a, b) => true;
                   getRuntimeTypeInfo(o) {}
                   setRuntimeTypeInfo(o, i) {}
                   eqNull(a) {}
                   eqNullB(a) {}""";
     } else if (uri.path.endsWith('_patch.dart')) {
       source = '';
     } else if (uri.path.endsWith('interceptors.dart')) {
       source = """class ObjectInterceptor {}
+                  class JSIndexable {
+                    get length;
+                  }
                   class JSArray {
-                    var length;
                     var removeLast;
                     var add;
                   }
                   class JSFixedArray {}
                   class JSExtendableArray {}
                   class JSString {
-                    var length;
                     var split;
                     var concat;
                     var toString;
                   }
                   class JSFunction {}
                   class JSInt {}
                   class JSDouble {}
                   class JSNumber {}
                   class JSNull {}
                   class JSBool {}
@@ skipping 30 matching lines @@
               new Uri.fromComponents(scheme: 'package', path: '/'),
               provider, handler);
   result.then((String code) {
     if (code == null) {
       throw 'Compilation failed';
     }
   }, onError: (e) {
       throw 'Compilation failed';
   });
 }