Stop waiting for service isolate before starting first DartApp

Fixes #405

R=kulakowski@chromium.org

Review URL: https://codereview.chromium.org/1336613002 .

mojo/dart/embedder/dart_controller.cc

diff --git a/mojo/dart/embedder/dart_controller.cc b/mojo/dart/embedder/dart_controller.cc
index ad86db9..985f752 100644
--- a/mojo/dart/embedder/dart_controller.cc
+++ b/mojo/dart/embedder/dart_controller.cc
@@ -690,8 +690,6 @@
 }
 
 bool DartController::RunDartScript(const DartControllerConfig& config) {
-  BlockForServiceIsolate();
-  CHECK(service_isolate_running_);
   const bool strict = strict_compilation_ || config.strict_compilation;
   Dart_Isolate isolate = CreateIsolateHelper(config.application_data,
                                              strict,