Dart: Adds a content handler and a test.

mojo/dart/embedder/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 action("generate_snapshot_bin") {
   deps = [
     "//dart/runtime/bin:gen_snapshot",
+    "//mojo/public/interfaces/application",
   ]
   inputs = [
     "snapshot.dart",
     "//dart/runtime/tools/create_snapshot_bin.py",
     "//mojo/dart/embedder/builtin.dart",
     "//mojo/public/dart/bindings.dart",
     "//mojo/public/dart/core.dart",
+    "//mojo/public/dart/src/application.dart",
     "//mojo/public/dart/src/buffer.dart",
     "//mojo/public/dart/src/client.dart",
     "//mojo/public/dart/src/codec.dart",
     "//mojo/public/dart/src/data_pipe.dart",
+    "//mojo/public/dart/src/drain_data.dart",
     "//mojo/public/dart/src/event_stream.dart",
     "//mojo/public/dart/src/handle.dart",
     "//mojo/public/dart/src/handle_watcher.dart",
     "//mojo/public/dart/src/interface.dart",
     "//mojo/public/dart/src/message.dart",
     "//mojo/public/dart/src/message_pipe.dart",
+    "//mojo/public/dart/src/service_provider.dart",
     "//mojo/public/dart/src/struct.dart",
     "//mojo/public/dart/src/timer_impl.dart",
     "//mojo/public/dart/src/timer_queue.dart",
     "//mojo/public/dart/src/types.dart",
   ]
   output = "$target_gen_dir/snapshot_gen.bin"
   outputs = [
     output,
   ]
 
@@ skipping 61 matching lines @@
 
   defines = []
   if (is_debug) {
     defines += [ "DEBUG" ]
   } else {
     defines += [ "NDEBUG" ]
   }
 
   include_dirs = [ "//dart/runtime" ]
 }