Dart: Adds a content handler and a test.

services/BUILD.gn

Index: services/BUILD.gn
diff --git a/services/BUILD.gn b/services/BUILD.gn
index 165f90750562aaecd7887f89f369c05189aa447d..b64f83491fc21598aec938f88e0c8f52a1d2eab4 100644
--- a/services/BUILD.gn
+++ b/services/BUILD.gn
@@ -3,6 +3,7 @@
 # found in the LICENSE file.
 
 import("//build/config/ui.gni")
+import("//mojo/public/mojo.gni")
 
 group("services") {
   deps = [
@@ -20,4 +21,8 @@ group("services") {
     "//services/view_manager",
     "//services/window_manager",
   ]
+
+  if (mojo_use_dart) {
+    deps += [ "//services/dart" ]
+  }
 }