Add a snapshot for docgen and use it in the build

utils/apidoc/docgen.gyp

 # 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' : {
     'script_suffix%': '',
   },
   'conditions' : [
     ['OS=="win"', {
@@ skipping 39 matching lines @@
           #
           # So we want this action to be re-run when a dart file
           # changes in this directory, or in the SDK library (we may
           # no longer need to list the files in ../../runtime/lib and
           # ../../runtime/bin, as most of them has moved to
           # ../../sdk/lib).
           #
           'inputs': [
             '<(PRODUCT_DIR)/<(EXECUTABLE_PREFIX)dart<(EXECUTABLE_SUFFIX)',
             '<(SHARED_INTERMEDIATE_DIR)/utils_wrapper.dart.snapshot',
+            '<(PRODUCT_DIR)/<(EXECUTABLE_PREFIX)/dart-sdk/bin/docgen<(script_suffix)',
             '<!@(["python", "../../tools/list_files.py", "\\.(css|ico|js|json|png|sh|txt|yaml|py)$", ".", "../../sdk/lib/_internal/dartdoc"])',
             '<!@(["python", "../../tools/list_files.py", "\\.dart$", ".", "../../sdk/lib", "../../runtime/lib", "../../runtime/bin"])',
             '../../sdk/bin/dart',
             '../../sdk/bin/dart.bat',
             '../../sdk/bin/dart2js',
             '../../sdk/bin/dart2js.bat',
             '../../tools/only_in_release_mode.py',
             # We sit inside the api_docs directory, so make sure it has run
             # before we do. Otherwise it might run later and delete us.
             '<(PRODUCT_DIR)/api_docs/index.html',
           ],
           'outputs': [
             '<(PRODUCT_DIR)/api_docs/docgen/index.json',
           ],
           'action': [
             'python',
             '../../tools/only_in_release_mode.py',
             '<@(_outputs)',
             '--',
-            '<(PRODUCT_DIR)/<(EXECUTABLE_PREFIX)dart<(EXECUTABLE_SUFFIX)',
-            '--old_gen_heap_size=1024',
-            '--package-root=<(PRODUCT_DIR)/packages/',
-            '../../pkg/docgen/bin/docgen.dart',
+            '<(PRODUCT_DIR)/<(EXECUTABLE_PREFIX)dart-sdk/bin/docgen<(script_suffix)',
             '--out=<(PRODUCT_DIR)/api_docs/docgen',
             '--json',
             '--include-sdk',
             '--package-root=<(PRODUCT_DIR)/packages',
             '--exclude-lib=async_helper',
             '--exclude-lib=expect',
             '--exclude-lib=docgen',
             '../../pkg',          
           ],
           'message': 'Running docgen: <(_action)',
         },
       ],
     }
   ],
 }