Emit patched_sdk in the root output directory.

runtime/vm/vm.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.
 
 {
   'variables': {
     'gen_source_dir': '<(SHARED_INTERMEDIATE_DIR)',
     'libgen_in_cc_file': '../lib/libgen_in.cc',
     'builtin_in_cc_file': '../bin/builtin_in.cc',
     'async_cc_file': '<(gen_source_dir)/async_gen.cc',
@@ skipping 1272 matching lines @@
             '<(gen_source_dir)/patches/internal_patch.dart',
             '<(gen_source_dir)/patches/io_patch.dart',
             '<(gen_source_dir)/patches/isolate_patch.dart',
             '<(gen_source_dir)/patches/math_patch.dart',
             '<(gen_source_dir)/patches/mirrors_patch.dart',
             '<(gen_source_dir)/patches/profiler_patch.dart',
             '<(gen_source_dir)/patches/vmservice_patch.dart',
           ],
           'outputs': [
             # Instead of listing all outputs we list a single well-known one.
-            '<(gen_source_dir)/patched_sdk/lib/core/core.dart',
+            '<(PRODUCT_DIR)/patched_sdk/lib/core/core.dart',
           ],
           'action': [
             'python',
             '../tools/patch_sdk.py',
             '--dart-executable',
             '<(PRODUCT_DIR)/<(EXECUTABLE_PREFIX)dart_bootstrap<(EXECUTABLE_SUFFIX)',
             'vm',
             '../sdk',
             '<(gen_source_dir)/patches',
-            '<(gen_source_dir)/patched_sdk',
+            '<(PRODUCT_DIR)/patched_sdk',
           ],
         },
       ],
     },
     {
       'variables': {
         'library_name': 'async',
         'library_uri': 'dart:async',
       },
       'target_name': 'generate_<(library_name)_library_patch',
@@ skipping 370 matching lines @@
             '--output',
             '<(gen_source_dir)/patches/<(library_name)_patch.dart',
             '<@(_sources)',
           ],
           'message': 'Generating <(library_uri) patch.',
         },
       ],
     },
   ]
 }