Optimize length access on all JSIndexable things.

tests/utils/recursive_import_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.
 
 // Test of "recursive" imports using the dart2js compiler API.
 
 import "package:expect/expect.dart";
 import 'dart:async';
 import '../../sdk/lib/_internal/compiler/compiler.dart';
 import 'dart:uri';
@@ skipping 18 matching lines @@
 class LinkedHashMap {}
 getRuntimeTypeInfo(o) {}
 setRuntimeTypeInfo(o, i) {}
 eqNull(a) {}
 eqNullB(a) {}
 class JSInvocationMirror {}  // Should be in helper.
 """;
 
 const INTERCEPTORS_LIB = """
 library interceptors;
+class JSIndexable {
+  get length;
+}
 class JSArray {
-  get length => null;
   removeLast() => null;
   add(x) { }
 }
 class JSString {
-  get length => null;
   split(x) => null;
   concat(x) => null;
   toString() => null;
 }
 """;
 
 const String RECURSIVE_MAIN = """
 library fisk;
 import 'recurse/fisk.dart';
 main() {}
@@ skipping 55 matching lines @@
     Expect.isNull(code);
     Expect.isTrue(10 < count);
     // Two warnings for each time RECURSIVE_MAIN is read, except the
     // first time.
     Expect.equals(2 * (count - 1), warningCount);
     Expect.equals(1, errorCount);
   }, onError: (e) {
       throw 'Compilation failed';
   });
 }