Fix incremental kernel generator.

pkg/front_end/lib/src/incremental_kernel_generator_impl.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 'package:front_end/file_system.dart';
 import 'package:front_end/incremental_kernel_generator.dart';
 import 'package:front_end/src/base/api_signature.dart';
@@ skipping 83 matching lines @@
       FileState entryLibrary = await _fsState.getFile(_entryPoint);
 
       List<LibraryCycle> cycles = _logger.run('Compute library cycles', () {
         List<LibraryCycle> cycles = entryLibrary.topologicalOrder;
         _logger.writeln('Computed ${cycles.length} cycles.');
         return cycles;
       });
 
       CanonicalName nameRoot = new CanonicalName.root();
       DillTarget dillTarget =
-          new DillTarget(new Ticker(isVerbose: false), _uriTranslator);
+          new DillTarget(new Ticker(isVerbose: false), _uriTranslator, "vm");

[scheglov, 2017/05/22 16:05:06]

Why not use new VmTarget() directly?
Why should we go through additional map lookup and use magic names anyway?

[ahe, 2017/05/22 16:09:56]

Because I'm assuming that this is an option that should be specified on the
command line.

 
       List<_LibraryCycleResult> results = [];
       await _logger.runAsync('Compute results for cycles', () async {
         for (LibraryCycle cycle in cycles) {
           _LibraryCycleResult result =
               await _compileCycle(nameRoot, dillTarget, cycle);
           results.add(result);
         }
       });
 
@@ skipping 184 matching lines @@
   /// TODO(scheglov) Use API signatures.
   /// TODO(scheglov) Or use tree shaking and compute signatures of outlines.
   final String signature;
 
   /// Kernel libraries for libraries in the [cycle].  Bodies of dependencies
   /// are not included, but but references to those dependencies are included.
   final List<Library> kernelLibraries;
 
   _LibraryCycleResult(this.cycle, this.signature, this.kernelLibraries);
 }