Update test scripts to run under the Dart 1.6 vm.

tools/testing/dart/browser_controller.dart

Index: tools/testing/dart/browser_controller.dart
diff --git a/tools/testing/dart/browser_controller.dart b/tools/testing/dart/browser_controller.dart
index 37eb07f5a359fb39f37aeb1bae8846d3ae1d1325..d76e43796a6b2d5923cb6ad34282bfcc002f5fdb 100644
--- a/tools/testing/dart/browser_controller.dart
+++ b/tools/testing/dart/browser_controller.dart
@@ -1308,9 +1308,11 @@ class BrowserTestingServer {
       if (request.uri.path.startsWith(driverPath)) {
         var browserId = request.uri.path.substring(driverPath.length + 1);
         textResponse = getDriverPage(browserId);
+        request.response.headers.set('Content-Type', 'text/html');
       } else if (request.uri.path.startsWith(nextTestPath)) {
         var browserId = request.uri.path.substring(nextTestPath.length + 1);
         textResponse = getNextTest(browserId);
+        request.response.headers.set('Content-Type', 'text/plain');
       } else {
         // /favicon.ico requests
       }