Validate that root paths are absolute and normalized.

pkg/analyzer/lib/file_system/physical_file_system.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 physical_file_system;
 
 import 'dart:async';
 import 'dart:core' hide Resource;
 import 'dart:io' as io;
 
@@ skipping 15 matching lines @@
   static final PhysicalResourceProvider INSTANCE =
       new PhysicalResourceProvider(null);
 
   /**
    * The name of the directory containing plugin specific subfolders used to
    * store data across sessions.
    */
   static final String SERVER_DIR = ".dartServer";
 
   final AbsolutePathContext absolutePathContext =
-      new AbsolutePathContext(io.Platform.pathSeparator);
+      new AbsolutePathContext(io.Platform.isWindows);
 
   PhysicalResourceProvider(String fileReadMode(String s)) {
     if (fileReadMode != null) {
       FileBasedSource.fileReadMode = fileReadMode;
     }
   }
 
   @override
   Context get pathContext => io.Platform.isWindows ? windows : posix;
 
@@ skipping 173 matching lines @@
   bool operator ==(other) {
     if (runtimeType != other.runtimeType) {
       return false;
     }
     return path == other.path;
   }
 
   @override
   String toString() => path;
 }