Put tree-shaking behind a flag in dartk.

pkg/kernel/lib/target/targets.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.targets;
 
 import '../ast.dart';
 
 import 'vm.dart';
 import 'vmcc.dart';
 import 'flutter.dart';
 
 final List<String> targetNames = targets.keys.toList();
 
 class TargetFlags {
   bool strongMode;
-  TargetFlags({this.strongMode: false});
+  bool treeShake;
+
+  TargetFlags({this.strongMode: false, this.treeShake: false});
 }
 
 typedef Target _TargetBuilder(TargetFlags flags);
 
 final Map<String, _TargetBuilder> targets = <String, _TargetBuilder>{
   'none': (TargetFlags flags) => new NoneTarget(flags),
   'vm': (TargetFlags flags) => new VmTarget(flags),
   'vmcc': (TargetFlags flags) => new VmClosureConvertedTarget(flags),
   'flutter': (TargetFlags flags) => new FlutterTarget(flags),
 };
@@ skipping 33 matching lines @@
 class NoneTarget extends Target {
   final TargetFlags flags;
 
   NoneTarget(this.flags);
 
   bool get strongMode => flags.strongMode;
   String get name => 'none';
   List<String> get extraRequiredLibraries => <String>[];
   void transformProgram(Program program) {}
 }