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

utils/compiler/BUILD.gn

 # 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.
 
 import("../../build/compiled_action.gni")
+import("../../build/prebuilt_dart_sdk.gni")
+import("../../utils/generate_patch_sdk.gni")
 import("../create_timestamp.gni")
 import("../application_snapshot.gni")
 
 create_timestamp_file("dart2js_files_stamp") {
   path = rebase_path("../../pkg/compiler/lib")
   output = "$target_gen_dir/dart2js_files.stamp"
 }
 
 create_timestamp_file("runtime_lib_files_stamp") {
   path = rebase_path("../../runtime/lib")
@@ skipping 52 matching lines @@
   ]
 }
 
 application_snapshot("utils_wrapper") {
   deps = [
     ":dart2js_create_snapshot_entries",
   ]
   main_dart = "$target_gen_dir/utils_wrapper.dart"
   training_args = [ "--help" ]
 }
+
+generate_patched_sdk("patched_dart2js_sdk") {
+  mode = "dart2js"
+  deps = ["../../runtime/vm:patched_sdk"]
+  input_patches_dir = "../../sdk/lib/_internal/js_runtime/lib"
+  patched_sdk_dir = "patched_dart2js_sdk"
+}