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

pkg/kernel/lib/target/targets.dart

Index: pkg/kernel/lib/target/targets.dart
diff --git a/pkg/kernel/lib/target/targets.dart b/pkg/kernel/lib/target/targets.dart
index 4ac768d08ba341e0ff30fbac9f3347f2ca9bddb4..7386e9e44812e08d56d16b837e3c75f36c667455 100644
--- a/pkg/kernel/lib/target/targets.dart
+++ b/pkg/kernel/lib/target/targets.dart
@@ -6,6 +6,7 @@ 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';
@@ -30,6 +31,7 @@ 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),