Don't remove things, especially isolates, from a map if they're not there

Source/bindings/core/dart/DartScriptDebugServer.h

 /*
  * Copyright (C) 2014 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 70 matching lines @@
     T remove(int handle)
     {
         T value = m_handleToValueMap.take(handle);
         m_valueToHandleMap.remove(value);
         return value;
     }
 
     int removeByValue(T value)
     {
         int handle = m_valueToHandleMap.take(value);
-        m_handleToValueMap.remove(handle);
+        // TODO(alanknight): This is clearly wrong. We should never call this
+        //  with an invalid value, but we are doing so sometimes in isolate
+        // shutdown. We need to track down the underlying cause, but this
+        // is a workaround.
+        if (handle != 0) {
+            m_handleToValueMap.remove(handle);
+        }
         return handle;
     }
 
     void copyValues(Vector<T>& values)
     {
         copyKeysToVector(m_valueToHandleMap, values);
     }
 
 private:
     int m_lastHandle;
@@ skipping 170 matching lines @@
     PageScriptDebugServer::ClientMessageLoop* m_clientMessageLoop;
 
     DartInjectedScriptManager* m_injectedScriptManager;
 
     int m_nextPageId;
 };
 
 }
 
 #endif // DartScriptDebugServer_h