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

tests/compiler/dart2js/compiler_helper.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 constant folding.
 
 library compiler_helper;
 
 import "dart:uri";
 
 import "../../../sdk/lib/_internal/compiler/implementation/elements/elements.dart"
        as lego;
 import "../../../sdk/lib/_internal/compiler/implementation/js_backend/js_backend.dart"
        as js;
 import "../../../sdk/lib/_internal/compiler/implementation/dart2jslib.dart"
        as leg;
 import "../../../sdk/lib/_internal/compiler/implementation/ssa/ssa.dart" as ssa;
 import "../../../sdk/lib/_internal/compiler/implementation/util/util.dart";
 import '../../../sdk/lib/_internal/compiler/implementation/source_file.dart';
 
 import "mock_compiler.dart";
 import "parser_helper.dart";
 
 String compile(String code, {String entry: 'main',
                              String coreSource: DEFAULT_CORELIB,
+                             String interceptorsSource: DEFAULT_INTERCEPTORSLIB,
                              bool enableTypeAssertions: false,
                              bool minify: false,
                              bool analyzeAll: false}) {
   MockCompiler compiler =
       new MockCompiler(enableTypeAssertions: enableTypeAssertions,
                        coreSource: coreSource,
+                       interceptorsSource: interceptorsSource,
                        enableMinification: minify);
   compiler.parseScript(code);
   lego.Element element = compiler.mainApp.find(buildSourceString(entry));
   if (element == null) return null;
   compiler.backend.enqueueHelpers(compiler.enqueuer.resolution);
   compiler.processQueue(compiler.enqueuer.resolution, element);
   var context = new js.JavaScriptItemCompilationContext();
   leg.WorkItem work = new leg.WorkItem(element, null, context);
   work.run(compiler, compiler.enqueuer.codegen);
   return compiler.enqueuer.codegen.lookupCode(element);
@@ skipping 79 matching lines @@
   final xRe = new RegExp('\\bx\\b');
   regexp = regexp.replaceAll(xRe, '(?:$anyIdentifier)');
   final spaceRe = new RegExp('\\s+');
   regexp = regexp.replaceAll(spaceRe, '(?:\\s*)');
   if (shouldMatch) {
     Expect.isTrue(new RegExp(regexp).hasMatch(generated));
   } else {
     Expect.isFalse(new RegExp(regexp).hasMatch(generated));
   }
 }