Dart: Remove unused library and example

mojo/common/dart/BUILD.gn

Index: mojo/common/dart/BUILD.gn
diff --git a/mojo/common/dart/BUILD.gn b/mojo/common/dart/BUILD.gn
deleted file mode 100644
index 72f23f2e2dc26ee7b688bc9668237b1085a966ff..0000000000000000000000000000000000000000
--- a/mojo/common/dart/BUILD.gn
+++ /dev/null
@@ -1,17 +0,0 @@
-# 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") {
-  libs = [ "lib/tracing_helper.dart" ]
-  sources = [
-    "lib/src/trace_provider_impl.dart",
-    "pubspec.yaml",
-  ]
-  deps = [
-    "//mojo/public/dart",
-    "//mojo/dart/packages",
-  ]
-}