Add support for compiling Dart via the FE in dart2js.

tests/compiler/dart2js/kernel/class_hierarchy_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.
 
 /// Test that the dart2js copy of [KernelVisitor] generates the expected class
 /// hierarchy.
 
 import 'package:compiler/src/commandline_options.dart' show Flags;
 import 'package:compiler/src/compiler.dart' show Compiler;
 import 'package:compiler/src/js_backend/backend.dart' show JavaScriptBackend;
@@ skipping 14 matching lines @@
         mMethod() {}
       }
       class C extends S with M {
         cMethod() {}
       }
       main() {}
       '''
   }, options: [
     Flags.analyzeOnly,
     Flags.analyzeAll,
-    Flags.useKernel
+    Flags.useKernelInSsa
   ]);
   test('mixin', () async {
     Uri mainUri = Uri.parse('memory:main.dart');
     await compiler.run(mainUri);
     LoadedLibraries libraries =
         await compiler.libraryLoader.loadLibrary(mainUri);
     compiler.processLoadedLibraries(libraries);
     JavaScriptBackend backend = compiler.backend;
     ir.Program program = backend.kernelTask.buildProgram(libraries.rootLibrary);
     ClosedWorldClassHierarchy hierarchy =
@@ skipping 26 matching lines @@
             hierarchy.getDispatchTarget(subClass, member.name), equals(member),
             reason: 'Unexpected dispatch target for ${member.name} '
                 'in $subClass');
       }
     }
 
     checkInheritance(classS, classC);
     checkInheritance(classM, classC);
   });
 }