Add vector_math and box2d as external Dart packages

mojo/dart/third_party/BUILD.gn

Index: mojo/dart/third_party/BUILD.gn
diff --git a/services/dart/test/echo/BUILD.gn b/mojo/dart/third_party/BUILD.gn
similarity index 52%
copy from services/dart/test/echo/BUILD.gn
copy to mojo/dart/third_party/BUILD.gn
index c484aa5dd85306253e5a62d22b6e6e4652dcd58c..8932e4c1588a62ef1046a0e14b131bf0dd17ab8d 100644
--- a/services/dart/test/echo/BUILD.gn
+++ b/mojo/dart/third_party/BUILD.gn
@@ -4,13 +4,17 @@
 
 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"
+}