Compile embedder unit test Dart to kernel (flutter/engine#7231)

As of the migration to Dart 2, it has been necessary to compile Dart to
kernel prior to execution. The embedder currently requires that the
resulting kernel file be named `kernel_blob.bin` and be located at the
root of the assets directory passed to the embedder API.

This patch updates the test_fixtures build rule to perform a kernel
compile using frontend_server, outputting `kernel_blob.bin` to
`fixtures/test_target_name` directory, and updates the embedder
unittests to specify the kernel file rather than the Dart source file.

Since the kernel compiler requires a `main()` function to be defined, it
also updates `simple_main.dart` from runtime_unittests to define
`main()` rather than `simple_main()`.

This also updates all existing sub-targets to be testonly.

This relands commit 4e4fb4608da95d198b0e796478462285ab974a3c, which was
reverted in commit 566db0ecb8f293bb9f7ff1fc39076b08336e0148. Rather than
running as prebuilt_dart_action, we use dart_action to ensure the
frontend snapshot it compatible with the VM on which it's executed.

engine/src/flutter/runtime/fixtures/simple_main.dart

@@ -2,6 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-void simple_main() {
+void main() {
   print("Hello");
 }

engine/src/flutter/shell/platform/embedder/fixtures/simple_main.dart

@@ -1 +1 @@
-void main() {}
+void main() {}

engine/src/flutter/shell/platform/embedder/tests/embedder_unittests.cc

@@ -22,17 +22,14 @@ TEST(EmbedderTest, CanLaunchAndShutdownWithValidProjectArgs) {
     return false;
   };
 
-  std::string main =
-      std::string(testing::GetFixturesPath()) + "/simple_main.dart";
-
   FlutterRendererConfig config = {};
   config.type = FlutterRendererType::kSoftware;
   config.software = renderer;
 
   FlutterProjectArgs args = {};
   args.struct_size = sizeof(FlutterProjectArgs);
-  args.assets_path = "";
-  args.main_path = main.c_str();
+  args.assets_path = testing::GetFixturesPath();
+  args.main_path = "";
   args.packages_path = "";
 
   FlutterEngine engine = nullptr;

engine/src/flutter/testing/testing.gni

@@ -2,18 +2,21 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//third_party/dart/build/dart/dart_action.gni")
+
+# Builds test fixtures for a unit test.
+#
+# Generates a directory structure containing an assets directory and the Dart
+# code to execute compiled to kernel, emitted to assets/kernel_blob.bin.
 template("test_fixtures") {
   testonly = true
 
   assert(defined(invoker.fixtures), "Test fixtures must be specified.")
 
+  fixtures_location = "$target_gen_dir/$target_name/assets"
+  fixtures_location_file = "$target_gen_dir/$target_name/test_fixtures_location.cc"
+
   fixtures_name_target_name = target_name + "_gen_fixtures_name"
-  fixtures_source_set_target_name = target_name + "_gen_fixtures_source_set"
-  fixtures_copy_target_name = target_name + "_copy_fixtures"
-
-  fixtures_location = "$target_gen_dir/fixtures"
-  fixtures_location_file = "$target_gen_dir/test_fixtures_location.cc"
-
   action(fixtures_name_target_name) {
     script = "$flutter_root/testing/build/gen_fixtures_location_symbol.py"
 
@@ -29,7 +32,9 @@ template("test_fixtures") {
     ]
   }
 
+  fixtures_source_set_target_name = target_name + "_gen_fixtures_source_set"
   source_set(fixtures_source_set_target_name) {
+    testonly = true
     sources = [
       fixtures_location_file,
     ]
@@ -39,16 +44,32 @@ template("test_fixtures") {
     ]
   }
 
-  copy(fixtures_copy_target_name) {
-    sources = invoker.fixtures
-    outputs = [
-      "$fixtures_location/{{source_file_part}}",
+  fixtures_kernel_target_name = target_name + "_kernel"
+  dart_action(fixtures_kernel_target_name) {
+    testonly = true
+    script = "$root_out_dir/frontend_server.dart.snapshot"
+
+    fixture_paths = []
+    foreach(fixture, invoker.fixtures) {
+      fixture_paths += [ rebase_path(fixture) ]
+    }
+    inputs = fixture_paths
+    outputs = ["$fixtures_location/kernel_blob.bin"]
+
+    args = [
+      "--sdk-root", rebase_path("$root_out_dir/flutter_patched_sdk"),
+      "--target", "flutter",
+      "--output-dill", rebase_path("$fixtures_location/kernel_blob.bin"),
+    ] + fixture_paths
+    deps = [
+      "//third_party/dart/utils/kernel-service:frontend_server"
     ]
   }
 
   group(target_name) {
+    testonly = true
     deps = [
-      ":$fixtures_copy_target_name",
+      ":$fixtures_kernel_target_name",
       ":$fixtures_source_set_target_name",
     ]
   }