Add support for compiling Dart via the FE in dart2js.

tests/compiler/dart2js/kernel/run_from_dill_test.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.
 
 // Test that we can compile from dill and run the generated code with d8.
 library dart2js.kernel.run_from_dill_test;
 
 import 'dart:async';
 import 'dart:io';
 
@@ skipping 49 matching lines @@
     entryPoint = Uri.parse('memory:main.dart');
     memorySourceFiles = SOURCE;
   }
 
   Uri dillFile =
       await generateDill(entryPoint, memorySourceFiles, printSteps: true);
   String output = uriPathToNative(dillFile.resolve('out.js').path);
   List<String> dart2jsArgs = [
     dillFile.toString(),
     '-o$output',
-    Flags.loadFromDill,
+    Flags.useKernel,
     Flags.disableTypeInference,
     Flags.disableInlining,
     Flags.enableAssertMessage
   ];
   print('Running: dart2js ${dart2jsArgs.join(' ')}');
 
   await dart2js.internalMain(dart2jsArgs);
 
   print('---- run from dill --------------------------------------------');
   ProcessResult runResult = Process.runSync(d8executable,
       ['sdk/lib/_internal/js_runtime/lib/preambles/d8.js', output]);
   String out = '${runResult.stderr}\n${runResult.stdout}';
   print('d8 output:');
   print(out);
   Expect.equals(0, runResult.exitCode);
   Expect.equals(OUTPUT, runResult.stdout.replaceAll('\r\n', '\n'));
 
   return ResultKind.success;
 }