Add basic support for a configuration file.

test/runner/pub_serve_test.dart

 // Copyright (c) 2015, 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.
 
 @TestOn("vm")
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
@@ skipping 13 matching lines @@
     d.file("pubspec.yaml", """
 name: myapp
 dependencies:
   barback: any
   test: {path: ${p.current}}
 transformers:
 - myapp:
     \$include: test/**_test.dart
 - test/pub_serve:
     \$include: test/**_test.dart
-dependency_overrides:
-  matcher: '0.12.0-alpha.0'
 """).create();
 
     d.dir("test", [
       d.file("my_test.dart", """
 import 'package:test/test.dart';
 
 void main() {
   test("test", () => expect(true, isTrue));
 }
 """)
@@ skipping 260 matching lines @@
 
     var test = runTest(['--pub-serve=54321', 'my_test.dart']);
     test.stdout.expect(containsInOrder([
       '-1: loading my_test.dart',
       'Failed to load "my_test.dart": When using "pub serve", all test files '
           'must be in test/.'
     ]));
     test.shouldExit(1);
   });
 }