Fix breaking change from next SDK

lib/src/paths.dart

 // Copyright (c) 2013, 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.
 
 /**
  * Holds path information that is used by the WebUI compiler to find files,
  * compute their output location and relative paths between them.
  */
 library web_ui.src.paths;
 
@@ skipping 123 matching lines @@
 
   /**
    * Transforms a [target] url seen in [src] (e.g. a Dart import, a .css href in
    * an HTML file, etc) into a corresponding url from the output file associated
    * with [src]. This will keep 'package:', 'dart:', path-absolute, and absolute
    * urls intact, but it will fix relative paths to walk from the output
    * directory back to the input directory. An exception will be thrown if
    * [target] is not under [_baseDir].
    */
   String transformUrl(String src, String target) {
-    if (new Uri.fromString(target).isAbsolute) return target;
+    if (Uri.parse(target).isAbsolute) return target;
     if (path.isAbsolute(target)) return target;
     return toUrl(path.normalize(path.relative(
           path.join(path.dirname(src), target), from: outputDirPath(src))));
   }
   
   /** Convert a OS specific path into a url. */
   static String toUrl(String relPath) =>
     (path.separator == '/') ? relPath : path.split(relPath).join('/');
 }
 
 /**
  * Returns a "mangled" name, with a prefix and [suffix] depending on the
  * compiler's settings. [forceSuffix] causes [suffix] to be appended even if
  * the compiler is not mangling names.
  */
 typedef String NameMangler(String name, String suffix, [bool forceSuffix]);