Dart: Fix bug in unbind() for Stubs.

mojo/public/platform/dart/dart_handle_watcher.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <cstdio>
 #include <cstring>
 #include <memory>
 #include <mutex>
 #include <set>
 #include <unordered_map>
@@ skipping 150 matching lines @@
     handle_ports_.push_back(port);
     handle_to_index_map_[handle] = index;
   }
 
   // Sanity check.
   MOJO_CHECK(wait_many_handles_.size() == handle_to_index_map_.size());
 }
 
 void HandleWatcherThreadState::RemoveHandle(MojoHandle handle) {
   auto it = handle_to_index_map_.find(handle);
-  MOJO_CHECK(it != handle_to_index_map_.end());
+
+  // Removal of a handle for an incoming event can race with the removal of
+  // a handle for an unsubscribe() call on the Dart MojoEventSubscription.
+  // This is not an error, so we ignore attempts to remove a handle that is not
+  // in the map.
+  if (it == handle_to_index_map_.end()) {
+    return;
+  }
   const intptr_t index = it->second;
   // We should never be removing the control handle.
   MOJO_CHECK(index != CONTROL_HANDLE_INDEX);
   RemoveHandleAtIndex(index);
 }
 
 void HandleWatcherThreadState::CloseHandle(MojoHandle handle,
                                            Dart_Port port,
                                            bool pruning) {
   MOJO_CHECK(!pruning || (port == ILLEGAL_PORT));
@@ skipping 400 matching lines @@
     control_handles.push_back(it.first);
   }
 
   for (auto it : control_handles) {
     StopLocked(it);
   }
 }
 
 }  // namespace dart
 }  // namespace mojo