Add support for building patched_sdk and platform.dill for dart2js:

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 '../core_types.dart';
 import '../transformations/treeshaker.dart' show ProgramRoot;
+import 'dart2js.dart';
 import 'flutter.dart';
 import 'vm.dart';
 import 'vmcc.dart';
 import 'vmreify.dart';
 
 final List<String> targetNames = targets.keys.toList();
 
 class TargetFlags {
   bool strongMode;
   bool treeShake;
   List<ProgramRoot> programRoots;
   Uri kernelRuntime;
 
   TargetFlags(
       {this.strongMode: false,
       this.treeShake: false,
       this.programRoots: const <ProgramRoot>[],
       this.kernelRuntime});
 }
 
 typedef Target _TargetBuilder(TargetFlags flags);
 
 final Map<String, _TargetBuilder> targets = <String, _TargetBuilder>{
   'none': (TargetFlags flags) => new NoneTarget(flags),
+  'dart2js': (TargetFlags flags) => new Dart2jsTarget(flags),

[ahe, 2017/04/27 11:48:31]

Since targets is public, you can add this target from outside package:kernel. I
think that's preferable as package:compiler is clearly owned by a team that's
different from the kernel team.

Potentially, all the targets could live outside package:kernel, but right now
people working on package:kernel are also primarily working on the targets
listed in this file.

[Siggi Cherem (dart-lang), 2017/04/28 21:37:20]

Done, actually gone :-) - I have no need to create one anymore.

   'vm': (TargetFlags flags) => new VmTarget(flags),
   'vmcc': (TargetFlags flags) => new VmClosureConvertedTarget(flags),
   'vmreify': (TargetFlags flags) => new VmGenericTypesReifiedTarget(flags),
   'flutter': (TargetFlags flags) => new FlutterTarget(flags),
 };
 
 Target getTarget(String name, TargetFlags flags) {
   var builder = targets[name];
   if (builder == null) return null;
   return builder(flags);
@@ skipping 44 matching lines @@
   final TargetFlags flags;
 
   NoneTarget(this.flags);
 
   bool get strongMode => flags.strongMode;
   String get name => 'none';
   List<String> get extraRequiredLibraries => <String>[];
   void performModularTransformations(Program program) {}
   void performGlobalTransformations(Program program) {}
 }