Clean up bogus codepath in ContextManager._computePackageUriResolver.

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:convert';
 import 'dart:core' hide Resource;
@@ skipping 697 matching lines @@
 
   /**
    * Compute the appropriate package URI resolver for [folder], and store
    * dependency information in [info]. Return `null` if no package map can
    * be computed.
    */
   UriResolver _computePackageUriResolver(Folder folder, ContextInfo info) {
     _cancelDependencySubscriptions(info);
     if (info.packageRoot != null) {
       info.packageMapInfo = null;
+      // TODO(paulberry): We shouldn't be using JavaFile here because it
+      // makes the code untestable (see dartbug.com/23909).
       JavaFile packagesDir = new JavaFile(info.packageRoot);
       Map<String, List<Folder>> packageMap = new Map<String, List<Folder>>();
       if (packagesDir.isDirectory()) {
         for (JavaFile file in packagesDir.listFiles()) {
           // Ensure symlinks in packages directory are canonicalized
           // to prevent 'type X cannot be assigned to type X' warnings
           String path;
           try {
             path = file.getCanonicalPath();
           } catch (e, s) {
             // Ignore packages that do not exist
             _instrumentationService.logException(e, s);
             continue;
           }
           Resource res = resourceProvider.getResource(path);
           if (res is Folder) {
             packageMap[file.getName()] = <Folder>[res];
           }
         }
         return new PackageMapUriResolver(resourceProvider, packageMap);
       }
-      //TODO(danrubel) remove this if it will never be called
-      return new PackageUriResolver([packagesDir]);
+      // The package root does not exist (or is not a folder).  Since
+      // [setRoots] ignores any package roots that don't exist (or aren't
+      // folders), the only way we should be able to get here is due to a race
+      // condition.  In any case, the package root folder is gone, so we can't
+      // resolve packages.
+      return null;
     } else {
       callbacks.beginComputePackageMap();
       if (packageResolverProvider != null) {
         UriResolver resolver = packageResolverProvider(folder);
         if (resolver != null) {
           return resolver;
         }
       }
       OptimizingPubPackageMapInfo packageMapInfo;
       ServerPerformanceStatistics.pub.makeCurrentWhile(() {
@@ skipping 458 matching lines @@
    */
   final List<AnalysisContext> removed;
 
   /**
    * Initialize a newly created event to indicate which contexts have changed.
    */
   ContextsChangedEvent({this.added: AnalysisContext.EMPTY_LIST,
       this.changed: AnalysisContext.EMPTY_LIST,
       this.removed: AnalysisContext.EMPTY_LIST});
 }