Type ReactDebugTool (#7586)
Type ReactDebugTool (#7586)

Flow doesn't really support the concept of variables that are non-null but only inside of a `__DEV__` block. There's an internal post about it ( https://www.facebook.com/groups/flowtype/permalink/1132437726804841/ ) and the conclusion is that we should force it to be non-null and trust the developer to put the proper DEV checks in place.
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