Add a build-time flag for enabling Dart in Sky

sky/engine/BUILD.gn

 # Copyright 2014 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("config.gni")
 if (is_clang) {
   import("//build/config/clang/clang.gni")
 }
 if (is_posix) {
   import("//build/config/gcc/gcc_version.gni")
@@ skipping 37 matching lines @@
 
   if (is_linux && cpu_arch == "arm") {
     # Due to a bug in gcc arm, we get warnings about uninitialized
     # timesNewRoman.unstatic.3258 and colorTransparent.unstatic.4879.
     cflags += [ "-Wno-uninitialized" ]
   }
 
   if (is_posix && gcc_version >= 49) {
     cflags += [ "-Wno-unused-but-set-variable" ]
   }
-
-  if (is_clang) {
-    if (sky_gc_plugin && clang_use_chrome_plugins) {
-      # TODO(GYP)
-      #'cflags': ['<!@(../../tools/clang/scripts/blink_gc_plugin_flags.sh enable-oilpan=<(sky_enable_oilpan) dump-graph=<(sky_gc_plugin_dump_graph))'],
-    }
-  }
 }
 
 # The follow configs apply to all targets except for unit tests, which rely on
 # static initializers.
 config("non_test_config") {
   cflags = []
 
   if (is_clang) {
     cflags += [ "-Wglobal-constructors" ]
   }
 }
 
 group("engine") {
   deps = [
     "//sky/engine/web",
     "//sky/engine/platform",
   ]
 }