Fix build.dart: sourceUrl used to be a String before the change to source_span

pkg/polymer/lib/src/build/runner.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.
 
 /// Definitions used to run the polymer linter and deploy tools without using
 /// pub serve or pub deploy.
 library polymer.src.build.runner;
 
 import 'dart:async';
 import 'dart:convert';
@@ skipping 328 matching lines @@
 
 /// Formatter that generates messages using a format that can be parsed
 /// by tools, such as the Dart Editor, for reporting error messages.
 String _jsonFormatter(LogEntry entry) {
   var kind = _kindFromEntry(entry);
   var span = entry.span;
   return JSON.encode((span == null)
       ? [{'method': kind, 'params': {'message': entry.message}}]
       : [{'method': kind,
           'params': {
-            'file': span.sourceUrl,
+            'file': span.sourceUrl.toString(),
             'message': entry.message,
             'line': span.start.line + 1,
             'charStart': span.start.offset,
             'charEnd': span.end.offset,
           }}]);
 }
 
 /// Formatter that generates messages that are easy to read on the console (used
 /// by default).
 String _consoleFormatter(LogEntry entry) {
   var kind = _kindFromEntry(entry);
   var useColors = stdioType(stdout) == StdioType.TERMINAL;
   var levelColor = (kind == 'error') ? _RED_COLOR : _MAGENTA_COLOR;
   var output = new StringBuffer();
   if (useColors) output.write(levelColor);
   output..write(kind)..write(' ');
   if (useColors) output.write(_NO_COLOR);
   if (entry.span == null) {
     output.write(entry.message);
   } else {
     output.write(entry.span.message(entry.message,
           color: useColors ? levelColor : null));
   }
   return output.toString();
 }
 
 const String _RED_COLOR = '\u001b[31m';
 const String _MAGENTA_COLOR = '\u001b[35m';
 const String _NO_COLOR = '\u001b[0m';