Put tree-shaking behind a flag in dartk.

pkg/kernel/lib/target/vm.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.
 library kernel.target.vm;
 
 import '../ast.dart';
+import '../class_hierarchy.dart';
 import '../core_types.dart';
 import '../transformations/continuation.dart' as cont;
 import '../transformations/erasure.dart';
 import '../transformations/insert_covariance_checks.dart';
 import '../transformations/insert_type_checks.dart';
 import '../transformations/mixin_full_resolution.dart' as mix;
 import '../transformations/sanitize_for_vm.dart';
 import '../transformations/setup_builtin_library.dart' as setup_builtin_library;
 import '../transformations/treeshaker.dart';
 import 'targets.dart';
@@ skipping 31 matching lines @@
         'dart:profiler',
         'dart:typed_data',
         'dart:vmservice_io',
         'dart:_vmservice',
         'dart:_builtin',
         'dart:nativewrappers',
         'dart:io',
       ];
 
   void transformProgram(Program program) {
-    var mixins = new mix.MixinFullResolution();
-    mixins.transform(program);
+    var mixins = new mix.MixinFullResolution()..transform(program);
 
     var hierarchy = mixins.hierarchy;
     var coreTypes = new CoreTypes(program);
 
     if (strongMode) {
       new InsertTypeChecks(hierarchy: hierarchy, coreTypes: coreTypes)
           .transformProgram(program);
       new InsertCovarianceChecks(hierarchy: hierarchy, coreTypes: coreTypes)
           .transformProgram(program);
     }
 
-    if (program.mainMethod != null) {
+    if (flags.treeShake) {
       new TreeShaker(program,
-              hierarchy: hierarchy, coreTypes: coreTypes,
+              hierarchy: hierarchy,
+              coreTypes: coreTypes,
               strongMode: strongMode)
           .transform(program);
+      hierarchy = null; // Hierarchy must be recomputed.
     }
 
     cont.transformProgram(program);
 
     // Repair `_getMainClosure()` function in dart:_builtin.
     setup_builtin_library.transformProgram(program);
 
     if (strongMode) {
       new Erasure().transform(program);
     }
 
     new SanitizeForVM().transform(program);
   }
 }