Dart: Removes all but native calls and the handle watcher from the snapshot.

services/dart/test/echo/BUILD.gn

Index: services/dart/test/echo/BUILD.gn
diff --git a/services/dart/test/echo/BUILD.gn b/services/dart/test/echo/BUILD.gn
index edce3a6023277dd6ad7966ab916f9205f812c400..54692a1bc194cc816b693729c5c2eced6693746a 100644
--- a/services/dart/test/echo/BUILD.gn
+++ b/services/dart/test/echo/BUILD.gn
@@ -10,7 +10,8 @@ dart_packaged_application("echo") {
     "main.dart",
   ]
   deps = [
-    "//services/dart/test:echo_service_interface",
+    "//mojo/public/dart",
     "//mojo/public/interfaces/application",
+    "//services/dart/test:echo_service_interface",
   ]
 }