Make it possible to write a message pipe endpoint's peer into it.

mojo/edk/system/core_test_base.cc

Index: mojo/edk/system/core_test_base.cc
diff --git a/mojo/edk/system/core_test_base.cc b/mojo/edk/system/core_test_base.cc
index d73706da464f6baffd47357416c4ddb9ec81f9d9..7df5fb79e410ac131b436b4a477724d7a9edb338 100644
--- a/mojo/edk/system/core_test_base.cc
+++ b/mojo/edk/system/core_test_base.cc
@@ -191,7 +191,10 @@ class MockDispatcher : public Dispatcher {
     mutex().AssertHeld();
   }
 
-  RefPtr<Dispatcher> CreateEquivalentDispatcherAndCloseImplNoLock() override {
+  RefPtr<Dispatcher> CreateEquivalentDispatcherAndCloseImplNoLock(
+      MessagePipe* /*message_pipe*/,
+      unsigned /*port*/) override {
+    CancelAllAwakablesNoLock();
     return Create(info_);
   }