Perform override and inheritance checks.

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.
 // VMOptions=
 // VMOptions=--print-object-histogram
 
 // Smoke test of the dart2js compiler API.
 library dummy_compiler;
 
 import 'dart:async';
@@ skipping 36 matching lines @@
 eqNullB(a) {}
 const proxy = 0;""";
     } else if (uri.path.endsWith('_patch.dart')) {
       source = '';
     } else if (uri.path.endsWith('interceptors.dart')) {
       source = """
 class Interceptor {
   operator==(other) {}
   get hashCode => throw 'Interceptor.hashCode not implemented.';
 }
-class JSIndexable {
-  get length {}
+abstract class JSIndexable {
+  get length;
 }
-class JSMutableIndexable {}
-class JSArray<E> implements JSIndexable {
+abstract class JSMutableIndexable {}
+abstract class JSArray<E> implements JSIndexable {
   JSArray() {}
   factory JSArray.typed(a) => a;
   var removeLast;
   var add;
 }
-class JSMutableArray extends JSArray {}
-class JSFixedArray extends JSMutableArray {}
-class JSExtendableArray extends JSMutableArray {}
+abstract class JSMutableArray extends JSArray {}
+abstract class JSFixedArray extends JSMutableArray {}
+abstract class JSExtendableArray extends JSMutableArray {}
 class JSString implements JSIndexable {
   var split;
   var concat;
   operator+(other) {}
   var toString;
+  get length => 0;
 }
 class JSFunction {}
 class JSInt {}
 class JSPositiveInt {}
 class JSUInt31 {}
 class JSUInt32 {}
 class JSDouble {}
 class JSNumber {}
 class JSNull {}
 class JSBool {}
@@ skipping 32 matching lines @@
               new Uri(scheme: 'package', path: '/'),
               provider, handler);
   result.then((String code) {
     if (code == null) {
       throw 'Compilation failed';
     }
   }, onError: (e) {
       throw 'Compilation failed';
   }).then(asyncSuccess);
 }