Remove v8 source dependency, enable more compiler warnings

dart/tools/gyp/configurations_make.gypi

 # Copyright (c) 2011, 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.
 
 {
   'variables': {
     'arm_cross_libc%': '/opt/codesourcery/arm-2009q1/arm-none-linux-gnueabi/libc',
     'dart_debug_optimization_level%': '2',
   },
   'target_defaults': {
     'configurations': {
       'Dart_Base': {
         'abstract': 1,
         'cflags': [
           '-Werror',
           '<@(common_gcc_warning_flags)',
           '-Wnon-virtual-dtor',
           '-Wvla',
           '-Wno-conversion-null',
-          # TODO(v8-team): Fix V8 build.
-          #'-Woverloaded-virtual',
+          '-Woverloaded-virtual',
           '-g3',
           '-ggdb3',
           # TODO(iposva): Figure out if we need to pass anything else.
           #'-ansi',
           '-fno-rtti',
           '-fno-exceptions',
           '-fvisibility=hidden',
           '-fvisibility-inlines-hidden',
         ],
       },
@@ skipping 50 matching lines @@
           '-fno-omit-frame-pointer',
         ],
       },
 
       'Dart_Release': {
         'cflags': [ '-O3', ],
       },
     },
   },
 }