Improve [relativize] for Windows.

dart/lib/compiler/implementation/util/uri_extras.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.
 
 #library('uri_extras');
 
 #import('dart:math');
 #import('dart:uri');
 
-String relativize(Uri base, Uri uri) {
-  if (base.scheme == 'file' &&
-      base.scheme == uri.scheme &&
+String relativize(Uri base, Uri uri, bool isWindows) {
+  if (!base.path.startsWith('/')) {
+    // Also throw an exception if [base] or base.path is null.
+    throw new IllegalArgumentException('Expected absolute path: ${base.path}');
+  }
+  if (!uri.path.startsWith('/')) {
+    // Also throw an exception if [uri] or uri.path is null.
+    throw new IllegalArgumentException('Expected absolute path: ${uri.path}');
+  }
+  bool equalsNCS(String a, String b) {
+    return a.toLowerCase() == b.toLowerCase();
+  }
+
+  String normalize(String path) {
+    if (isWindows) {
+      return path.toLowerCase();
+    } else {
+      return path;
+    }
+  }
+
+  if (equalsNCS(base.scheme, 'file') &&
+      equalsNCS(base.scheme, uri.scheme) &&
       base.userInfo == uri.userInfo &&
-      base.domain == uri.domain &&
+      equalsNCS(base.domain, uri.domain) &&
       base.port == uri.port &&
       uri.query == "" && uri.fragment == "") {
-    if (uri.path.startsWith(base.path)) {
+    if (normalize(uri.path).startsWith(normalize(base.path))) {
       return uri.path.substring(base.path.length);
     }
     List<String> uriParts = uri.path.split('/');
     List<String> baseParts = base.path.split('/');
     int common = 0;
     int length = min(uriParts.length, baseParts.length);
-    while (common < length && uriParts[common] == baseParts[common]) {
+    while (common < length &&
+           normalize(uriParts[common]) == normalize(baseParts[common])) {
       common++;
     }
+    if (common == 1 || (isWindows && common == 2)) {
+      // The first part will always be an empty string because the
+      // paths are absolute. On Windows, we must also consider drive
+      // letters or hostnames.
+      return uri.path;
+    }
     StringBuffer sb = new StringBuffer();
     for (int i = common + 1; i < baseParts.length; i++) {
       sb.add('../');
     }
     for (int i = common; i < uriParts.length - 1; i++) {
       sb.add('${uriParts[i]}/');
     }
     sb.add('${uriParts.last()}');
     return sb.toString();
   }
   return uri.toString();
 }