Dart: Reorganize files

mojo/dart/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.
 
 group("dart") {
   deps = [
-    "//mojo/dart/dart_snapshotter($host_toolchain)",
-    "//mojo/dart/http_load_test",
-    "//mojo/dart/observatory_test",
-    "//mojo/dart/packages/mojo_sdk",
-    "//mojo/dart/packages/mojo_services",
+    "//mojo/dart/embedder",
+    "//mojo/dart/packages",
   ]
 }
+
+group("tests") {
+  testonly = true
+  deps = [
+    "//mojo/dart/apptests",
+    "//mojo/dart/unittests",
+  ]
+}