watch for file system changes after context created

pkg/analysis_server/lib/src/context_manager.dart

 // Copyright (c) 2014, 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.
 
 library context.directory.manager;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:core' hide Resource;
 
@@ skipping 460 matching lines @@
    * Create a new empty context associated with [folder].
    */
   _ContextInfo _createContext(
       Folder folder, File pubspecFile, List<_ContextInfo> children) {
     _ContextInfo info = new _ContextInfo(
         folder, pubspecFile, children, normalizedPackageRoots[folder.path]);
     _contexts[folder] = info;
     info.changeSubscription = folder.changes.listen((WatchEvent event) {
       _handleWatchEvent(folder, info, event);
     });
-    UriResolver packageUriResolver = _computePackageUriResolver(folder, info);
-    info.context = addContext(folder, packageUriResolver);
-    info.context.name = folder.path;
+    try {
+      UriResolver packageUriResolver = _computePackageUriResolver(folder, info);
+      info.context = addContext(folder, packageUriResolver);
+      info.context.name = folder.path;
+    } catch (_) {
+      info.changeSubscription.cancel();
+      rethrow;
+    }
     return info;
   }
 
   /**
    * Potentially create a new context associated with the given [folder].
    *
    * If there are subfolders with 'pubspec.yaml' files, separate contexts are
    * created for them and excluded from the context associated with the
    * [folder].
    *
@@ skipping 340 matching lines @@
     return excludes(resource.path);
   }
 
   /**
    * Returns `true` if [path] is the pubspec file of this context.
    */
   bool isPubspec(String path) {
     return path == pubspecPath;
   }
 }