Fix lowercasing output filenames for dart:3320 as well as fix for co19 test

runtime/bin/path_impl.dart

 // Copyright (c) 2012, 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.
 
 class _Path implements Path {
   final String _path;
 
   const _Path(String source) : _path = source;
   _Path.fromNative(String source) : _path = _clean(source);
 
@@ skipping 146 matching lines @@
         segmentsToJoin.add('');
       }
     }
     return new Path(Strings.join(segmentsToJoin, '/'));
   }
 
   String toNativePath() {
     if (Platform.operatingSystem == 'windows') {
       String nativePath = _path;
       // Drop '/' before a drive letter.
-      if (nativePath.startsWith('/') && nativePath[2] == ':') {
+      if (nativePath.length > 3 &&
+          nativePath.startsWith('/') &&
+          nativePath[2] == ':') {
         nativePath = nativePath.substring(1);
       }
       nativePath = nativePath.replaceAll('/', '\\');
       return nativePath;
     }
     return _path;
   }
 
   List<String> segments() {
     List result = _path.split('/');
@@ skipping 30 matching lines @@
     if (pos < 0) return new Path('');
     while (pos > 0 && _path[pos - 1] == '/') --pos;
     return new Path((pos > 0) ? _path.substring(0, pos) : '/');
   }
 
   String get filename() {
     int pos = _path.lastIndexOf('/');
     return _path.substring(pos + 1);
   }
 }