Moves BUILD.gn config from root to runtime.

runtime/BUILD.gn

Index: runtime/BUILD.gn
===================================================================
--- runtime/BUILD.gn	(revision 41673)
+++ runtime/BUILD.gn	(working copy)
@@ -2,8 +2,33 @@
 # 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",
+  ]
+}
+
+
 static_library("libdart") {
-  configs += ["..:dart_config"]
+  configs += [":dart_config"]
   deps = [
     "vm:libdart_lib",
     "vm:libdart_vm",