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

Source/bindings/core/dart/DartScriptDebugServer.h

Index: Source/bindings/core/dart/DartScriptDebugServer.h
diff --git a/Source/bindings/core/dart/DartScriptDebugServer.h b/Source/bindings/core/dart/DartScriptDebugServer.h
index c65ac3a465cbbffe5f7aad739d78d5b8ca383b79..30228dc049d1ea307b58291ad9a44be0eb354d7a 100644
--- a/Source/bindings/core/dart/DartScriptDebugServer.h
+++ b/Source/bindings/core/dart/DartScriptDebugServer.h
@@ -88,7 +88,13 @@ public:
     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;
     }