Add vector_math and box2d as external Dart packages

mojo/dart/third_party/BUILD.gn

 # Copyright 2015 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.
 
 import("//mojo/public/dart/rules.gni")
 
-dartzip_packaged_application("echo") {
-  output_name = "dart_echo"
-  sources = [
-    "main.dart",
-  ]
+group("third_party") {
   deps = [
-    "//mojo/public/dart",
-    "//services/dart/test:echo_service_interface",
+    ":box2d",
+    ":vector_math",
   ]
 }
+
+dart_pkg("box2d") {
+  pkg_dir = "//third_party/dart-pkg/box2d"
+}
+
+dart_pkg("vector_math") {
+  pkg_dir = "//third_party/dart-pkg/vector_math"
+}