Fixes for updated third_party/android_tools.

tools/gyp/configurations_android.gypi

 # Copyright (c) 2012, 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.
 
 # Definitions for building standalone Dart binaries to run on Android.
 # This is mostly excerpted from:
 # http://src.chromium.org/viewvc/chrome/trunk/src/build/common.gypi
 
 {
   'variables': {
@@ skipping 191 matching lines @@
           }],
           ['_toolset=="host"', {
             'cflags': [ '-m32', '-pthread' ],
             'ldflags': [ '-m32', '-pthread' ],
           }],
         ],
       },  # Dart_Android_arm_Base
       'Dart_Android_arm64_Base': {
         'abstract': 1,
         'variables': {
-          'android_sysroot': '<(android_ndk_root)/platforms/android-L/arch-arm64',
+          'android_sysroot': '<(android_ndk_root)/platforms/android-21/arch-arm64',
           'android_ndk_include': '<(android_sysroot)/usr/include',
           'android_ndk_lib': '<(android_sysroot)/usr/lib',
         },
         'target_conditions': [
           ['_toolset=="target"', {
             'cflags': [
               '-fPIE',
               '--sysroot=<(android_sysroot)',
               '-I<(android_ndk_include)',
               '-I<(android_ndk_root)/sources/cxx-stl/stlport/stlport',
@@ skipping 30 matching lines @@
             ],
           }],
           ['_toolset=="host"', {
             'ldflags': [ '-pthread' ],
           }],
         ],
       },  # Dart_Android_arm64_Base
     },  # configurations
   },  # target_defaults
 }