Add firefox support.

lib/src/runner/browser/browser.dart

Index: lib/src/runner/browser/browser.dart
diff --git a/lib/src/runner/browser/browser.dart b/lib/src/runner/browser/browser.dart
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+++ b/lib/src/runner/browser/browser.dart
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+// 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.
+
+library test.runner.browser.browser;
+
+import 'dart:async';
+
+/// An interface for running browser instances.
+///
+/// This is intentionally coarse-grained: browsers are controlled primary from
+/// inside a single tab. Thus this interface only provides support for closing
+/// the browser and seeing if it closes itself.
+///
+/// Any errors starting or running the browser process are reported through
+/// [onExit].
+abstract class Browser {
+  /// A future that completes when the browser exits.
+  ///
+  /// If there's a problem starting or running the browser, this will complete
+  /// with an error.
+  Future get onExit;
+
+  /// Kills the browser process.
+  ///
+  /// Returns the same [Future] as [onExit], except that it won't emit
+  /// exceptions.
+  Future close();
+}