Add support for compiling Dart via the FE in dart2js.

tests/compiler/dart2js/kernel/closed_world_test.dart

 // Copyright (c) 2016, 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.
 
 // Tests that the closed world computed from [WorldImpact]s derived from kernel
 // is equivalent to the original computed from resolution.
 library dart2js.kernel.closed_world_test;
 
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/commandline_options.dart';
@@ skipping 57 matching lines @@
     memorySourceFiles = SOURCE;
   }
 
   asyncTest(() async {
     enableDebugMode();
     Compiler compiler = compilerFor(
         entryPoint: entryPoint,
         memorySourceFiles: memorySourceFiles,
         options: [
           Flags.analyzeOnly,
-          Flags.useKernel,
+          Flags.useKernelInSsa,
           Flags.enableAssertMessage
         ]);
     ElementResolutionWorldBuilder.useInstantiationMap = true;
     compiler.resolution.retainCachesForTesting = true;
     await compiler.run(entryPoint);
     compiler.resolutionWorldBuilder.closeWorld();
 
     JavaScriptBackend backend = compiler.backend;
     // Create a new resolution enqueuer and feed it with the [WorldImpact]s
     // computed from kernel through the [build] in `kernel_impact.dart`.
@@ skipping 81 matching lines @@
 
 ClosedWorld computeClosedWorld(DiagnosticReporter reporter,
     ResolutionEnqueuer enqueuer, ElementEnvironment elementEnvironment) {
   enqueuer.open(const ImpactStrategy(), elementEnvironment.mainFunction,
       elementEnvironment.libraries);
   enqueuer.forEach((WorkItem work) {
     enqueuer.applyImpact(work.run(), impactSource: work.element);
   });
   return enqueuer.worldBuilder.closeWorld();
 }