Added --exclude-lib=docgen so that it does throw an error about markdown when building.

utils/apidoc/apidoc.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 69 matching lines @@
             '<(PRODUCT_DIR)/<(EXECUTABLE_PREFIX)dart<(EXECUTABLE_SUFFIX)',
             '--package-root=<(PRODUCT_DIR)/packages/',
             'apidoc.dart',
             '--out=<(PRODUCT_DIR)/api_docs',
             '--version=<!@(["python", "../../tools/print_version.py"])',
             '--package-root=<(PRODUCT_DIR)/packages',
             '--mode=static',
             '--exclude-lib=analyzer_experimental',
             '--exclude-lib=browser',
             '--exclude-lib=dartdoc',
+            '--exclude-lib=docgen',
             '--exclude-lib=expect',
             '--exclude-lib=http',
             '--exclude-lib=http_server',
             '--exclude-lib=oauth2',
             '--exclude-lib=pathos',
             '--exclude-lib=scheduled_test',
             '--exclude-lib=stack_trace',
             '--exclude-lib=webdriver',
             '--exclude-lib=yaml',
             '--include-lib=matcher',
             '--extra-lib=pkg/unittest/lib/mock.dart',
           ],
           'message': 'Running apidoc: <(_action)',
         },
       ],
     }
   ],
 }