Check PackageMapUriResolver constructor arguments.

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 'package:analyzer/file_system/file_system.dart';
@@ skipping 293 matching lines @@
         if (child.shortName == PACKAGES_NAME) {
           continue;
         }
         _addSourceFiles(changeSet, child, info);
       }
     }
   }
 
   /**
    * Compute the appropriate package URI resolver for [folder], and store
-   * dependency information in [info].
+   * dependency information in [info]. Return `null` if no package map can
+   * be computed.
    */
   UriResolver _computePackageUriResolver(Folder folder, _ContextInfo info) {
-    UriResolver packageUriResolver;
     if (info.packageRoot != null) {
       info.packageMapDependencies = new Set<String>();
-      packageUriResolver =
-          new PackageUriResolver([new JavaFile(info.packageRoot)]);
+      return new PackageUriResolver([new JavaFile(info.packageRoot)]);
     } else {
       beginComputePackageMap();
       PackageMapInfo packageMapInfo =
           _packageMapProvider.computePackageMap(folder);
       endComputePackageMap();
       info.packageMapDependencies = packageMapInfo.dependencies;
-      packageUriResolver =
-          new PackageMapUriResolver(resourceProvider, packageMapInfo.packageMap);
+      if (packageMapInfo.packageMap == null) {
+        return null;
+      }
+      return new PackageMapUriResolver(
+          resourceProvider,
+          packageMapInfo.packageMap);
       // TODO(paulberry): if any of the dependencies is outside of [folder],
       // we'll need to watch their parent folders as well.
     }
-    return packageUriResolver;
   }
 
   /**
    * Create a new empty context associated with [folder].
    */
   _ContextInfo _createContext(Folder folder, List<_ContextInfo> children) {
     _ContextInfo info =
         new _ContextInfo(folder, children, normalizedPackageRoots[folder.path]);
     _contexts[folder] = info;
     info.changeSubscription = folder.changes.listen((WatchEvent event) {
@@ skipping 322 matching lines @@
     return excludes(resource.path);
   }
 
   /**
    * Returns `true` if [path] is the pubspec file of this context.
    */
   bool isPubspec(String path) {
     return path == pubspecPath;
   }
 }