Add browser path options to testing scripts.

tools/testing/dart/launch_browser.dart

Index: tools/testing/dart/launch_browser.dart
diff --git a/tools/testing/dart/launch_browser.dart b/tools/testing/dart/launch_browser.dart
index 3637906cda56cd5375ef0505424a9102738aad7b..047182fa29bc9f80efb2ace998fc13543a1739da 100644
--- a/tools/testing/dart/launch_browser.dart
+++ b/tools/testing/dart/launch_browser.dart
@@ -26,14 +26,15 @@ void main() {
     printHelp();
     return;
   }
+  var name = args[0];
 
-  if (!Browser.supportedBrowser(args[0])) {
+  if (!Browser.supportedBrowser(name)) {
     print("Specified browser not supported");
     printHelp();
     return;
   }
 
-  var browser = new Browser.byName(args[0]);
+  var executable = Locations.getBrowserExecutable(name, {});
+  var browser = new Browser.byName(name, executable);
   browser.start(args[1]);
-
 }