Add configuration support for choosing test paths.

lib/src/runner/configuration/values.dart

 // Copyright (c) 2016, 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.
 
 import 'dart:io';
 import 'dart:math' as math;
 
 import '../../util/io.dart';
 
 /// The default number of test suites to run at once.
 ///
 /// This defaults to half the available processors, since presumably some of
 /// them will be used for the OS and other processes.
 final defaultConcurrency = math.max(1, Platform.numberOfProcessors ~/ 2);
 
 /// The reporters supported by the test runner.
 const allReporters = const ["compact", "expanded", "json"];
 
 /// The default reporter.
 final defaultReporter = inTestTests
     ? 'expanded'
     : (Platform.isWindows ? 'expanded' : 'compact');
+
+/// The default filename pattern.
+///
+/// This is stored here so that we don't have to recompile it multiple times.
+final defaultFilename = new Glob("*_test.dart");