Add firefox support.

lib/src/runner/browser/chrome.dart

Index: lib/src/runner/browser/chrome.dart
diff --git a/lib/src/runner/browser/chrome.dart b/lib/src/runner/browser/chrome.dart
index c21ec8cfdbff127de66858f42a80b536fe5757df..fc4e232920e60dbc875b5f29550c94aab2d4f0c5 100644
--- a/lib/src/runner/browser/chrome.dart
+++ b/lib/src/runner/browser/chrome.dart
@@ -10,6 +10,7 @@ import 'dart:io';
 import 'package:path/path.dart' as p;
 
 import '../../util/io.dart';
+import 'browser.dart';
 
 // TODO(nweiz): move this into its own package?
 // TODO(nweiz): support other browsers.
@@ -20,7 +21,7 @@ import '../../util/io.dart';
 /// constructed, and is killed when [close] is called.
 ///
 /// Any errors starting or running the process are reported through [onExit].
-class Chrome {
+class Chrome implements Browser {
   /// The underlying process.
   Process _process;
 
@@ -30,10 +31,6 @@ class Chrome {
   /// well-isolated.
   String _dir;
 
-  /// 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 => _onExitCompleter.future;
   final _onExitCompleter = new Completer();
 
@@ -80,10 +77,6 @@ class Chrome {
         .catchError(_onExitCompleter.completeError);
   }
 
-  /// Kills the browser process.
-  ///
-  /// Returns the same [Future] as [onExit], except that it won't emit
-  /// exceptions.
   Future close() {
     _onProcessStarted.then((_) => _process.kill());