Dart: Remove unused library and example

benchmarks/dart_handler_running/BUILD.gn

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//mojo/public/dart/rules.gni")
 
 dart_pkg("dart_handler_running") {
   apps = [ [
         "dart_handler_running",
         "lib/main.dart",
       ] ]
   sources = [
     "pubspec.yaml",
   ]
   deps = [
-    "//mojo/common/dart",
     "//mojo/dart/packages",
     "//mojo/public/dart",
   ]
 }