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

tests/compiler/dart2js/mock_compiler.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.
 
 library mock_compiler;
 
 import 'dart:uri';
 
 import '../../../sdk/lib/_internal/compiler/compiler.dart' as api;
 import '../../../sdk/lib/_internal/compiler/implementation/dart2jslib.dart'
@@ skipping 34 matching lines @@
   abstract class JavaScriptIndexingBehavior {}
   class JSInvocationMirror {}
   S() {}
   assertHelper(a){}
   throwNoSuchMethod(obj, name, arguments, expectedArgumentNames) {}''';
 
 const String DEFAULT_INTERCEPTORSLIB = r'''
   class JSArray {
     var length;
     operator[](index) {}
+    var add;
   }
   class JSString {
     var length;
+    operator[](index) {}
   }
   class JSNumber {
   }
   class JSInt {
   }
   class JSDouble {
   }
   class JSNull {
   }
   class JSBool {
@@ skipping 158 matching lines @@
   }
 
   // The mock library doesn't need any patches.
   Uri resolvePatchUri(String dartLibraryName) => null;
 
   Script readScript(Uri uri, [ScriptTag node]) {
     SourceFile sourceFile = sourceFiles[uri.toString()];
     if (sourceFile == null) throw new ArgumentError(uri);
     return new Script(uri, sourceFile);
   }
+
+  Element findInScopeContainer(ScopeContainerElement scope, SourceString name) {
+    Element element = scope.localLookup(name);
+    return element != null ? element : new UnresolvedElement(name, scope);
+  }
 }
 
 void compareWarningKinds(String text, expectedWarnings, foundWarnings) {
   var fail = (message) => Expect.fail('$text: $message');
   HasNextIterator<MessageKind> expected =
       new HasNextIterator(expectedWarnings.iterator);
   HasNextIterator<WarningMessage> found =
       new HasNextIterator(foundWarnings.iterator);
   while (expected.hasNext && found.hasNext) {
     Expect.equals(expected.next(), found.next().message.kind);
@@ skipping 36 matching lines @@
   operator []=(Node node, Element element) {
     map[node] = element;
   }
 
   operator [](Node node) => map[node];
 
   void remove(Node node) {
     map.remove(node);
   }
 }