Add support for compiling Dart via the FE in dart2js.

tests/compiler/dart2js/dill_loader_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.
 
 import 'dart:async';
 import 'dart:io';
 import 'memory_compiler.dart';
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/commandline_options.dart';
 import 'package:compiler/src/common_elements.dart';
@@ skipping 44 matching lines @@
       ..setExitCodeOnProblem = true
       ..verify = true;
 
     List<int> kernelBinary =
         serializeProgram(await kernelForProgram(uri, options));
     CompilerImpl compiler = compilerFor(
         entryPoint: entryPoint,
         memorySourceFiles: {'main.dill': kernelBinary},
         diagnosticHandler: diagnostics,
         outputProvider: output,
-        options: [Flags.loadFromDill]);
+        options: [Flags.previewDart2]);
     await compiler.setupSdk();
     await compiler.libraryLoader.loadLibrary(entryPoint);
 
     Expect.equals(0, diagnostics.errors.length);
     Expect.equals(0, diagnostics.warnings.length);
 
     ElementEnvironment environment =
         compiler.frontendStrategy.elementEnvironment;
     LibraryEntity library = environment.lookupLibrary(uri);
     Expect.isNotNull(library);
     ClassEntity clss = environment.lookupClass(library, 'ListLiteralTest');
     Expect.isNotNull(clss);
     var member = environment.lookupClassMember(clss, 'testMain');
     Expect.isNotNull(member);
   });
 }