Dart: Adds a content handler and a test.

services/dart/test/BUILD.gn

Index: services/dart/test/BUILD.gn
diff --git a/sky/services/testing/BUILD.gn b/services/dart/test/BUILD.gn
similarity index 66%
copy from sky/services/testing/BUILD.gn
copy to services/dart/test/BUILD.gn
index affd0c34be07bdbe1fe7064ed1d73c9195a40fe6..bf656140b2715be4c72bf957c78cab9ed76be62f 100644
--- a/sky/services/testing/BUILD.gn
+++ b/services/dart/test/BUILD.gn
@@ -4,12 +4,8 @@
 
 import("//mojo/public/tools/bindings/mojom.gni")
 
-mojom("bindings") {
+mojom("echo_service") {
   sources = [
-    "test_harness.mojom",
-  ]
-
-  deps = [
-    "//mojo/services/input_events/public/interfaces",
+    "echo_service.mojom",
   ]
 }