Do not apply optimizations based on an element if that element cannot be resolved.

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 21 matching lines @@
                   class Dynamic_ {}
                   class Null {}
                   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 JSArray {}
-                  class JSString {}
+                  class JSArray {
+                    var length;
+                    var removeLast;
+                    var add;
+                  }
+                  class JSString {
+                    var length;
+                    var split;
+                    var concat;
+                  }
                   class JSFunction {}
                   class JSInt {}
                   class JSDouble {}
                   class JSNumber {}
                   class JSNull {}
                   class JSBool {}
                   var getInterceptor;""";
     } else if (uri.path.endsWith('js_helper.dart')) {
       source = 'library jshelper; class JSInvocationMirror {}';
     } else if (uri.path.endsWith('isolate_helper.dart')) {
@@ skipping 18 matching lines @@
 
 main() {
   String code = compile(new Uri.fromComponents(scheme: 'main'),
                         new Uri.fromComponents(scheme: 'lib', path: '/'),
                         new Uri.fromComponents(scheme: 'package', path: '/'),
                         provider, handler).value;
   if (code == null) {
     throw 'Compilation failed';
   }
 }