Create an apptesting framework for dart.

services/dart/dart_apptests/BUILD.gn

Index: services/dart/dart_apptests/BUILD.gn
diff --git a/services/dart/test/pingpong/BUILD.gn b/services/dart/dart_apptests/BUILD.gn
similarity index 51%
copy from services/dart/test/pingpong/BUILD.gn
copy to services/dart/dart_apptests/BUILD.gn
index 0b693d3513f5b99d9182c5a27873d5fa410061b6..f9cf331d890e7454cddc181cfd2b69d95496deb4 100644
--- a/services/dart/test/pingpong/BUILD.gn
+++ b/services/dart/dart_apptests/BUILD.gn
@@ -4,14 +4,21 @@
 
 import("//mojo/public/dart/rules.gni")
 
-dart_packaged_application("pingpong") {
-  output_name = "dart_pingpong"
+dart_packaged_application("dart_apptests") {
   sources = [
     "main.dart",
+    "echo_apptests.dart",
+    "pingpong_apptests.dart",
   ]
   deps = [
+    "//mojo/dart/apptest",
+    "//services/dart/test:echo_service_interface",
     "//services/dart/test:pingpong_service_interface",
-    "//mojo/public/interfaces/application",
+  ]
+  datadeps = [
+    "//services/dart/test/echo",
+    "//services/dart/test/pingpong",
+    "//services/dart/test/pingpong_target",
   ]
   strict = is_debug
 }