Revert "Emit warning on import of dart:mirrors."

pkg/compiler/lib/src/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';
 
 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 ArgumentError('Expected absolute path: ${base.path}');
+  }
+  if (!uri.path.startsWith('/')) {
+    // Also throw an exception if [uri] or uri.path is null.
+    throw new ArgumentError('Expected absolute path: ${uri.path}');
+  }
   bool equalsNCS(String a, String b) {
     return a.toLowerCase() == b.toLowerCase();
   }
 
-  if (!equalsNCS(base.scheme, uri.scheme) ||
-      equalsNCS(base.scheme, 'dart') ||
-      equalsNCS(base.scheme, 'package')) {
-    return uri.toString();
-  }
-
-  if (!equalsNCS(base.scheme, 'file')) {
-    isWindows = false;
-  }
-
   String normalize(String path) {
     if (isWindows) {
       return path.toLowerCase();
     } else {
       return path;
     }
   }
 
-  if (base.userInfo == uri.userInfo &&
+  if (equalsNCS(base.scheme, 'file') &&
+      equalsNCS(base.scheme, uri.scheme) &&
+      base.userInfo == uri.userInfo &&
       equalsNCS(base.host, uri.host) &&
       base.port == uri.port &&
       uri.query == "" && uri.fragment == "") {
     if (normalize(uri.path).startsWith(normalize(base.path))) {
       return uri.path.substring(base.path.lastIndexOf('/') + 1);
     }
 
-    if (!base.path.startsWith('/') ||
-        !uri.path.startsWith('/')) {
-      return uri.toString();
-    }
-
     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 &&
            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.
       if (baseParts.length > common + 1) {
         // Avoid using '..' to go to the root, unless we are already there.
         return uri.path;
       }
     }
     StringBuffer sb = new StringBuffer();
     for (int i = common + 1; i < baseParts.length; i++) {
       sb.write('../');
     }
     for (int i = common; i < uriParts.length - 1; i++) {
       sb.write('${uriParts[i]}/');
     }
     sb.write('${uriParts.last}');
     return sb.toString();
   }
   return uri.toString();
 }