Dart: Adds a content handler and a test.

services/dart/test/BUILD.gn

Index: services/dart/test/BUILD.gn
diff --git a/examples/js/maps/BUILD.gn b/services/dart/test/BUILD.gn
similarity index 71%
copy from examples/js/maps/BUILD.gn
copy to services/dart/test/BUILD.gn
index d10a973fdd751d8f3306b05f300286161e28fc21..bf656140b2715be4c72bf957c78cab9ed76be62f 100644
--- a/examples/js/maps/BUILD.gn
+++ b/services/dart/test/BUILD.gn
@@ -4,14 +4,8 @@
 
 import("//mojo/public/tools/bindings/mojom.gni")
 
-group("maps") {
-  deps = [
-    ":bindings",
-  ]
-}
-
-mojom("bindings") {
+mojom("echo_service") {
   sources = [
-    "geocoder.mojom",
+    "echo_service.mojom",
   ]
 }