Dart: Adds a content handler and a test.

mojo/dart/embedder/BUILD.gn

Index: mojo/dart/embedder/BUILD.gn
diff --git a/mojo/dart/embedder/BUILD.gn b/mojo/dart/embedder/BUILD.gn
index 0b670ddc3fe0cd5b06fb598ac982dec4bbd7b7d9..5efcdbcbdb8455d84fd5ff70985a9d330ffecc36 100644
--- a/mojo/dart/embedder/BUILD.gn
+++ b/mojo/dart/embedder/BUILD.gn
@@ -5,6 +5,7 @@
 action("generate_snapshot_bin") {
   deps = [
     "//dart/runtime/bin:gen_snapshot",
+    "//mojo/public/interfaces/application",
   ]
   inputs = [
     "snapshot.dart",
@@ -12,16 +13,19 @@ action("generate_snapshot_bin") {
     "//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",