Optimize length access on all JSIndexable things.

tests/compiler/dart2js/value_range_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.
 
 import "package:expect/expect.dart";
 import 'compiler_helper.dart';
 
 const int REMOVED = 0;
 const int ABOVE_ZERO = 1;
 const int BELOW_LENGTH = 2;
@@ skipping 204 matching lines @@
   class List {
     List([int length]);
   }
   abstract class Map {}
   class Closure {}
   class Null {}
   class Dynamic_ {}
   bool identical(Object a, Object b) {}''';
 
 const String INTERCEPTORSLIB_WITH_MEMBERS = r'''
-  class JSIndexable {}
+  abstract class JSIndexable {
+    get length;
+  }
   class JSArray implements JSIndexable {
     var length;
     var removeLast;
     operator[] (_) {}
   }
   class JSMutableArray extends JSArray {}
   class JSFixedArray extends JSMutableArray {}
   class JSExtendableArray extends JSMutableArray {}
   class JSString implements JSIndexable {
     var length;
@@ skipping 67 matching lines @@
       break;
   }
 }
 
 
 main() {
   for (int i = 0; i < TESTS.length;  i += 2) {
     expect(TESTS[i], TESTS[i + 1]);
   }
 }