Fix minor path handling issue in path package.

pkg/path/lib/path.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.
 
 /// A comprehensive, cross-platform path manipulation library.
 library path;
 
 import 'dart:io' as io;
 
 /// An internal builder for the current OS so we can provide a straight
@@ skipping 420 matching lines @@
     // find a path from `from` to `path`.
     if (this.isRelative(path) && this.isAbsolute(from)) {
       throw new ArgumentError('Unable to find a path to "$path" from "$from".');
     }
 
     var fromParsed = _parse(from)..normalize();
     var pathParsed = _parse(path)..normalize();
 
     // If the root prefixes don't match (for example, different drive letters
     // on Windows), then there is no relative path, so just return the absolute
-    // one.
-    // TODO(rnystrom): Drive letters are case-insentive on Windows. Should
-    // handle "C:\" and "c:\" being the same root.
-    if (fromParsed.root != pathParsed.root) return pathParsed.toString();
+    // one. In Windows, drive letters are case-insenstive and we allow
+    // calculation of relative paths, even if a path has not been normalized.
+    if (fromParsed.root != pathParsed.root &&
+        ((fromParsed.root ==  null || pathParsed.root == null) ||
+          fromParsed.root.toLowerCase().replaceAll('/', '\\') !=
+          pathParsed.root.toLowerCase().replaceAll('/', '\\'))) {
+      return pathParsed.toString();
+    }
 
     // Strip off their common prefix.
     while (fromParsed.parts.length > 0 && pathParsed.parts.length > 0 &&
            fromParsed.parts[0] == pathParsed.parts[0]) {
       fromParsed.parts.removeAt(0);
       fromParsed.separators.removeAt(0);
       pathParsed.parts.removeAt(0);
       pathParsed.separators.removeAt(0);
     }
 
@@ skipping 182 matching lines @@
       newParts.add('.');
     }
 
     // Canonicalize separators.
     var newSeparators = [];
     newSeparators.insertRange(0, newParts.length, style.separator);
 
     parts = newParts;
     separators = newSeparators;
 
+    // Normalize the Windows root if needed.
+    if (root != null && style == Style.windows) {
+      root = root.replaceAll('/', '\\');
+    }
     removeTrailingSeparators();
   }
 
   String toString() {
     var builder = new StringBuffer();
     if (root != null) builder.add(root);
     for (var i = 0; i < parts.length; i++) {
       builder.add(parts[i]);
       builder.add(separators[i]);
     }
@@ skipping 15 matching lines @@
     // If there is no dot, or it's the first character, like '.bashrc', it
     // doesn't count.
     if (lastDot <= 0) return [file, ''];
 
     return [file.substring(0, lastDot), file.substring(lastDot)];
   }
 
   _ParsedPath clone() => new _ParsedPath(
       style, root, new List.from(parts), new List.from(separators));
 }