Add factory constructor to browser controller.

tools/testing/dart/browser_controller.dart

Index: tools/testing/dart/browser_controller.dart
===================================================================
--- tools/testing/dart/browser_controller.dart	(revision 23334)
+++ tools/testing/dart/browser_controller.dart	(working copy)
@@ -52,7 +52,21 @@
 
   // We use this to gracefully handle double calls to close.
   bool underTermination = false;
+  
+  Browser();
 
+  factory Browser.byName(String name) {
+    if (name == 'ff' || name == 'firefox') {
+      return new FireFox();
+    } else if (name == 'chrome') {
+      return new Chrome();
+    } else if (name == 'safari') {
+      return new Safari();
+    } else {
+      throw "Non supported browser";
+    }
+  }
+
   void _logEvent(String event) {
     String toLog = "$this ($id) - ${new DateTime.now()}: $event \n";
     if (debugPrint) print("usageLog: $toLog");
@@ -719,16 +733,7 @@
   }
 
   Browser getInstance() {
-    var browser;
-    if (browserName == "chrome") {
-      browser = new Chrome();
-    } else if (browserName == "ff") {
-      browser = new Firefox();
-    } else if (browserName == "safari") {
-      browser = new Safari();
-    } else {
-      throw "Non supported browser for browser controller";
-    }
+    var browser = new Browser.byName(browserName);
     browser.logger = logger;
     return browser;
   }