Moves BUILD.gn config from root to runtime.

BUILD.gn

Index: BUILD.gn
===================================================================
--- BUILD.gn	(revision 41673)
+++ BUILD.gn	(working copy)
@@ -1,28 +0,0 @@
-# Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
-# for details. All rights reserved. Use of this source code is governed by a
-# BSD-style license that can be found in the LICENSE file.
-
-
-config("dart_config") {
-  defines = []
-  if (is_debug) {
-    defines += ["DEBUG"]
-  } else {
-    defines += ["NDEBUG"]
-  }
-
-  cflags = [
-    "-Werror",
-    "-Wall",
-    "-Wextra", # Also known as -W.
-    "-Wno-unused-parameter",
-    "-Wnon-virtual-dtor",
-    "-Wvla",
-    "-Wno-conversion-null",
-    "-Woverloaded-virtual",
-    "-g3",
-    "-ggdb3",
-    "-fno-rtti",
-    "-fno-exceptions",
-  ]
-}