Add location information to watchers to make them more debuggable.

lib/testing/render_test.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.
 
 /**
  * This is a helper for run.sh. We try to run all of the Dart code in one
  * instance of the Dart VM to reduce warm-up time.
  */
 library web_ui.testing.render_test;
 
 import 'dart:io';
 import 'dart:math' show min;
 import 'package:pathos/path.dart' as path;
 import 'package:unittest/unittest.dart';
 import 'package:web_ui/dwc.dart' as dwc;
 
 void renderTests(String baseDir, String inputDir, String expectedDir,
     String outDir, {List<String> arguments, String script, String pattern,
     bool deleteDir: true}) {
   if (arguments == null) arguments = new Options().arguments;
   if (script == null) script = new Options().script;
 
-  var filePattern = new RegExp(pattern != null ? pattern : '.');
+  var filePattern = new RegExp(pattern != null ? pattern
+      : (arguments.length > 0 ? arguments.removeAt(0) : '.'));
 
   var scriptDir = path.absolute(path.dirname(script));
   baseDir = path.join(scriptDir, baseDir);
   inputDir = path.join(scriptDir, inputDir);
   expectedDir = path.join(scriptDir, expectedDir);
   outDir = path.join(scriptDir, outDir);
 
   var paths = new Directory(inputDir).listSync()
       .where((f) => f is File).map((f) => f.path)
       .where((p) => p.endsWith('_test.html') && filePattern.hasMatch(p));
@@ skipping 52 matching lines @@
         var outPath = path.join(outDir, '$filename.txt');
         var expectedPath = path.join(expectedDir, '$filename.txt');
         new File(outPath).writeAsStringSync(output);
         var expected = new File(expectedPath).readAsStringSync();
         expect(output, expected,
           reason: 'unexpected output for <$filename>');
       });
     }
   }
 }