Add support for compiling Dart via the FE in dart2js.

tests/compiler/dart2js/kernel/helper.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.
 
 import 'dart:async';
 
 import 'package:compiler/src/compiler.dart' show Compiler;
 import 'package:compiler/src/elements/elements.dart'
     show Element, LibraryElement;
 import 'package:compiler/src/js_backend/backend.dart' as js
     show JavaScriptBackend;
 import 'package:compiler/src/commandline_options.dart' show Flags;
 import 'package:test/test.dart';
 
 import '../memory_compiler.dart';
 
 Future<String> compile(String code,
     {dynamic lookup: 'main',
-    bool useKernel: true,
+    bool useKernelInSsa: true,
     bool disableTypeInference: true,
     List<String> extraOptions: const <String>[]}) async {
   List<String> options = <String>[
     Flags.disableInlining,
   ];
   if (disableTypeInference) options.add(Flags.disableTypeInference);
-  if (useKernel) options.add(Flags.useKernel);
+  if (useKernelInSsa) options.add(Flags.useKernelInSsa);
   options.addAll(extraOptions);
 
   if (lookup is String && lookup != 'main' && !code.contains('main')) {
     code = "$code\n\nmain() => $lookup;";
   }
   CompilationResult result = await runCompiler(
       memorySourceFiles: {'main.dart': code}, options: options);
   expect(result.isSuccess, isTrue);
   Compiler compiler = result.compiler;
   LibraryElement mainApp =
@@ skipping 14 matching lines @@
 /// The function to check at the end is given by [lookup]. If [lookup] is a
 /// String, then the generated code for a top-level element named [lookup] is
 /// checked. Otherwise, [lookup] is a function that takes a [Compiler] and
 /// returns an [Element], and the returned [Element] is checked.
 Future check(String code,
     {dynamic lookup: 'main',
     bool disableTypeInference: true,
     List<String> extraOptions: const <String>[]}) async {
   var original = await compile(code,
       lookup: lookup,
-      useKernel: false,
+      useKernelInSsa: false,
       disableTypeInference: disableTypeInference,
       extraOptions: extraOptions);
   var kernel = await compile(code,
       lookup: lookup,
-      useKernel: true,
+      useKernelInSsa: true,
       disableTypeInference: disableTypeInference,
       extraOptions: extraOptions);
   expect(kernel, original);
 }