pkg/observe: wrapMicrotask should return the result the provided testCase

pkg/observe/lib/src/microtask.dart

Index: pkg/observe/lib/src/microtask.dart
diff --git a/pkg/observe/lib/src/microtask.dart b/pkg/observe/lib/src/microtask.dart
index e56fb623d6385b76ce5f785899e202b90d80624d..19d5e6009c0b88a7d8208a34b2a155beddf815e6 100644
--- a/pkg/observe/lib/src/microtask.dart
+++ b/pkg/observe/lib/src/microtask.dart
@@ -49,11 +49,11 @@ List<Function> _pending = [];
  * Wraps the [testCase] in a zone that supports [performMicrotaskCheckpoint],
  * and returns the test case.
  */
-wrapMicrotask(void testCase()) {
+wrapMicrotask(testCase()) {
   return () {
-    runZonedExperimental(() {
+    return runZonedExperimental(() {
       try {
-        testCase();
+        return testCase();
       } finally {
         performMicrotaskCheckpoint();
       }