Adds GN build files for building in GN based projects.

BUILD.gn

Index: BUILD.gn
===================================================================
--- BUILD.gn	(revision 0)
+++ BUILD.gn	(revision 0)
@@ -0,0 +1,28 @@
+# 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",
+  ]
+}