Add a snapshot for docgen and use it in the build

utils/compiler/compiler.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': {
     'dart_dir': '../..',
   },
   'targets': [
     {
       'target_name': 'dart2js',
       'type': 'none',
       'dependencies': [
         '../../runtime/dart-runtime.gyp:dart',
         '../../pkg/pkg.gyp:pkg_packages',
       ],
       'actions': [
         {
           'action_name': 'generate_snapshots',
           'inputs': [
             '<(PRODUCT_DIR)/<(EXECUTABLE_PREFIX)dart<(EXECUTABLE_SUFFIX)',
             '../../sdk/lib/_internal/libraries.dart',
-            '<!@(["python", "../../tools/list_files.py", "\\.dart$", "../../sdk/lib/_internal/compiler", "../../runtime/lib", "../../sdk/lib/_internal/dartdoc"])',
+            '<!@(["python", "../../tools/list_files.py", "\\.dart$", "../../sdk/lib/_internal/compiler", "../../runtime/lib", "../../sdk/lib/_internal/dartdoc", "../../pkg/docgen"])',
             'create_snapshot.dart',
             '<(SHARED_INTERMEDIATE_DIR)/packages.stamp',
             '../../tools/VERSION',
           ],
           'outputs': [
             '<(SHARED_INTERMEDIATE_DIR)/utils_wrapper.dart.snapshot',
             '<(SHARED_INTERMEDIATE_DIR)/dart2js.dart.snapshot',
           ],
           'action': [
             '<(PRODUCT_DIR)/<(EXECUTABLE_PREFIX)dart<(EXECUTABLE_SUFFIX)',
             'create_snapshot.dart',
             '--output_dir=<(SHARED_INTERMEDIATE_DIR)',
             '--dart2js_main=sdk/lib/_internal/compiler/implementation/dart2js.dart',
             '--dartdoc_main=sdk/lib/_internal/dartdoc/bin/dartdoc.dart',
+            '--docgen_main=pkg/docgen/bin/docgen.dart',
             '--package_root=<(PRODUCT_DIR)/packages/',
           ],
         },
       ],
     },
   ],
 }