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

pkg/kernel/lib/target/dart2js.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.dart2js;
+
+import '../ast.dart';
+import 'targets.dart';
+
+/// Specializes the kernel IR for dart2js
+class Dart2jsTarget extends Target {
+  final TargetFlags flags;
+
+  Dart2jsTarget(this.flags);
+
+  String get name => 'dart2js';
+  bool get strongMode => flags.strongMode;
+
+  List<String> get extraRequiredLibraries => const <String>[
+        'dart:async',
+        'dart:collection',
+        'dart:mirrors',
+        'dart:_native_typed_data',
+        'dart:_internal',
+        'dart:_js_helper',
+        'dart:_interceptors',
+        'dart:_foreign_helper',
+        'dart:_js_mirrors',
+        'dart:_js_names',
+        'dart:_js_embedded_names',
+        'dart:_isolate_helper',
+      ];
+
+  void performModularTransformations(Program program) {}
+  void performGlobalTransformations(Program program) {}
+}