Add runAsync interceptor.

tests/lib/async/intercept_run_async1_test.dart

Index: tests/lib/async/intercept_run_async1_test.dart
diff --git a/tests/lib/async/run_zoned1_test.dart b/tests/lib/async/intercept_run_async1_test.dart
similarity index 56%
copy from tests/lib/async/run_zoned1_test.dart
copy to tests/lib/async/intercept_run_async1_test.dart
index 71af2346c69ce7add4f4f8c5b030b2ca02385c73..c2bc9c18c5f2052d5aeff50286a0567a69d61bf0 100644
--- a/tests/lib/async/run_zoned1_test.dart
+++ b/tests/lib/async/intercept_run_async1_test.dart
@@ -6,6 +6,10 @@ import "package:expect/expect.dart";
 import 'dart:async';
 
 main() {
-  // Make sure `runZoned` returns the result of a synchronous call.
-  Expect.equals(499, runZonedExperimental(() => 499));
+  // Test that runZoned returns the result of executing the body.
+  var result = runZonedExperimental(() => 499,
+    onRunAsync: (f) {
+      Expect.fail("Unexpected invocation.");
+    });
+  Expect.equals(499, result);
 }