Add patch file support to Dart embedder

mojo/dart/embedder/test/BUILD.gn

Index: mojo/dart/embedder/test/BUILD.gn
diff --git a/mojo/dart/embedder/test/BUILD.gn b/mojo/dart/embedder/test/BUILD.gn
index 78c1d2a7b9e2484448c69eecf68b78e36503fb04..ee05ee5352c871b178b8dd1845f29128f5aff154 100644
--- a/mojo/dart/embedder/test/BUILD.gn
+++ b/mojo/dart/embedder/test/BUILD.gn
@@ -35,18 +35,24 @@ action("generate_test_snapshot_bin") {
     output,
   ]
 
-  gen_snapshot_dir = get_label_info(
-      "//dart/runtime/bin:gen_snapshot($host_toolchain)", "root_out_dir")
+  gen_snapshot_dir =
+      get_label_info("//dart/runtime/bin:gen_snapshot($host_toolchain)",
+                     "root_out_dir")
   script = "//dart/runtime/tools/create_snapshot_bin.py"
   builtin_path = rebase_path("//mojo/dart/embedder/builtin.dart")
   bindings_path = rebase_path("//mojo/public/dart/bindings.dart")
   core_path = rebase_path("//mojo/public/dart/core.dart")
   args = [
-    "--executable", rebase_path("$gen_snapshot_dir/gen_snapshot"),
-    "--package_root", rebase_path("$root_gen_dir"),
-    "--script", rebase_path("test_snapshot.dart"),
-    "--output_bin", rebase_path(output, root_build_dir),
-    "--target_os", os,
+    "--executable",
+    rebase_path("$gen_snapshot_dir/gen_snapshot"),
+    "--package_root",
+    rebase_path("$root_gen_dir"),
+    "--script",
+    rebase_path("test_snapshot.dart"),
+    "--output_bin",
+    rebase_path(output, root_build_dir),
+    "--target_os",
+    os,
     "--url_mapping=dart:mojo_bindings,$bindings_path",
     "--url_mapping=dart:mojo_builtin,$builtin_path",
     "--url_mapping=dart:mojo_core,$core_path",
@@ -69,14 +75,17 @@ action("generate_test_snapshot_file") {
 
   script = "//dart/runtime/tools/create_snapshot_file.py"
   args = [
-    "--input_bin", rebase_path("$target_gen_dir/test_snapshot_gen.bin"),
-    "--input_cc", rebase_path("//mojo/dart/embedder/snapshot.cc.tmpl"),
-    "--output", rebase_path(output),
+    "--input_bin",
+    rebase_path("$target_gen_dir/test_snapshot_gen.bin"),
+    "--input_cc",
+    rebase_path("//mojo/dart/embedder/snapshot.cc.tmpl"),
+    "--output",
+    rebase_path(output),
   ]
 }
 
 source_set("dart_controller_for_test") {
-  testonly=true
+  testonly = true
   sources = [
     "$target_gen_dir/test_snapshot.cc",
   ]
@@ -93,6 +102,7 @@ test("dart_unittests") {
     "validation_unittest.cc",
   ]
   deps = [
+    ":async_helper",
     ":dart_controller_for_test",
     ":dart_to_cpp_unittests",
     ":expect",
@@ -128,6 +138,15 @@ copy("validation_test_input_parser") {
   ]
 }
 
+copy("async_helper") {
+  sources = [
+    "//mojo/dart/testing/async_helper.dart",
+  ]
+  outputs = [
+    "{{source_gen_dir}}/{{source_file_part}}",
+  ]
+}
+
 source_set("dart_to_cpp_unittests") {
   testonly = true
   sources = [