Fix typos and error handling in setup_dart_analyzer

sky/tools/setup_dart_analyzer

 #!/usr/bin/env python
 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import argparse
 import subprocess
 import sys
 import os
 import yaml
 
 
 SKY_TOOLS_DIR = os.path.dirname(os.path.abspath(__file__))
 SRC_ROOT = os.path.dirname(os.path.dirname(SKY_TOOLS_DIR))
 SKY_SDK_DIR = os.path.join(SRC_ROOT, 'sky', 'sdk')
 SKY_PUBSPEC = os.path.join(SKY_SDK_DIR, 'pubspec.yaml')
 SKY_PUBSPEC_LOCK = os.path.join(SKY_SDK_DIR, 'pubspec.lock')
 SDK_EXT = os.path.join(SKY_SDK_DIR, 'lib', '_sdkext')
 
 SDK_EXT_TEMPLATE = '''{
   "dart:sky": "%(build_dir)s/gen/dart-pkg/sky/sdk_ext/dart_sky.dart",
-  "dart:sky.internals": "%(build_dir)s/gen/dart-pkg/sky/sky_internals.dart",
-  "dart:sky_builtin_natvies": "%(build_dir)s../sdk_ext/builtin_natives.dart"
+  "dart:sky.internals": "%(build_dir)s/gen/dart-pkg/sky/sdk_ext/sky_internals.dart",
+  "dart:sky_builtin_natvies": "%(build_dir)s/gen/dart-pkg/sky/sdk_ext/builtin_natives.dart"
 }'''
 
 def version_for_pubspec(pubspec_path):
     with open(pubspec_path, 'r') as stream:
         dependency_spec = yaml.load(stream)
         return dependency_spec['version']
 
 
 def entry_for_dependency(dart_pkg_dir, dependency):
     dependency_path = os.path.join(dart_pkg_dir, dependency)
     version = version_for_pubspec(os.path.join(dependency_path, 'pubspec.yaml'))
     return {
         'description': {
             'path': os.path.relpath(dependency_path, SKY_SDK_DIR),
             'relative': True,
         },
         'source': 'path',
         'version': version,
     }
 
 
 def main():
     parser = argparse.ArgumentParser(description='Adds files to the source tree to make the dart analyzer happy')
     parser.add_argument('build_dir', type=str, help='Path the build directory to use for build artifacts')
     args = parser.parse_args()
 
     dart_pkg_dir = os.path.join(args.build_dir, 'gen', 'dart-pkg')
+
+    if not os.path.exists(dart_pkg_dir):
+        print 'Cannot find Dart pacakges at "%s".' % dart_pkg_dir
+        print 'Did you run `ninja -C %s sky` ?' % os.path.relpath(args.build_dir, os.getcwd())
+        return 1
+
     packages = {}
 
     with open(SKY_PUBSPEC, 'r') as stream:
         spec = yaml.load(stream)
         for dependency in spec['dependencies'].keys():
             packages[dependency] = entry_for_dependency(dart_pkg_dir, dependency)
 
     lock = { 'packages': packages }
     with open(SKY_PUBSPEC_LOCK, 'w') as stream:
         yaml.dump(lock, stream=stream, default_flow_style=False)
 
     with open(SDK_EXT, 'w') as stream:
         rebased_build_dir = os.path.relpath(args.build_dir, os.path.dirname(SDK_EXT))
         stream.write(SDK_EXT_TEMPLATE % { 'build_dir': rebased_build_dir })
 
 
 if __name__ == '__main__':
     sys.exit(main())