Add the ability to run test suites in parallel.

lib/src/backend/live_test_controller.dart

Index: lib/src/backend/live_test_controller.dart
diff --git a/lib/src/backend/live_test_controller.dart b/lib/src/backend/live_test_controller.dart
index 71731b6f5cf8a31feac0517f63a4e92c9254983d..fd35458f871fa3e405eec9b655fc65b2ae36c391 100644
--- a/lib/src/backend/live_test_controller.dart
+++ b/lib/src/backend/live_test_controller.dart
@@ -159,6 +159,9 @@ class LiveTestController {
   Future _run() {
     if (_runCalled) {
       throw new StateError("LiveTest.run() may not be called more than once.");
+    } else if (_isClosed) {
+      throw new StateError("LiveTest.run() may not be called for a closed "
+          "test.");
     }
     _runCalled = true;