Add support for compiling Dart via the FE in dart2js.

pkg/compiler/lib/src/js_backend/element_strategy.dart

 // Copyright (c) 2017, 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 dart2js.js_backend.element_strategy;
 
 import '../backend_strategy.dart';
 import '../closure.dart' show ClosureConversionTask, ClosureTask;
 import '../common.dart';
 import '../common/codegen.dart';
@@ skipping 52 matching lines @@
 
   @override
   WorkItemBuilder createCodegenWorkItemBuilder(ClosedWorld closedWorld) {
     return new ElementCodegenWorkItemBuilder(
         _compiler.backend, closedWorld, _compiler.options);
   }
 
   @override
   SsaBuilder createSsaBuilder(CompilerTask task, JavaScriptBackend backend,
       SourceInformationStrategy sourceInformationStrategy) {
-    return _compiler.options.useKernel
+    return _compiler.options.useKernelInSsa
         ? new RastaSsaBuilder(task, backend, sourceInformationStrategy)
         : new SsaAstBuilder(task, backend, sourceInformationStrategy);
   }
 
   SourceInformationStrategy get sourceInformationStrategy {
     return _sourceInformationStrategy ??= createSourceInformationStrategy(
         generateSourceMap: _compiler.options.generateSourceMap,
         useMultiSourceInfo: _compiler.options.useMultiSourceInfo,
         useNewSourceInfo: _compiler.options.useNewSourceInfo);
   }
@@ skipping 76 matching lines @@
 
   MemberElement get element => resolvedAst.element;
 
   WorldImpact run() {
     registry = new CodegenRegistry(_closedWorld.elementEnvironment, element);
     return _backend.codegen(this, _closedWorld);
   }
 
   String toString() => 'CodegenWorkItem(${resolvedAst.element})';
 }