Emit warning when a module writes global state

[hershi]

When initializing dependencies, we evaluate each module in isolation, and if the module evaluation fails, we roll back its changes and continue with the next module in line. This can cause a change of behavior when running the prepacked code compared to the standard code, in case of badly behaved modules that have implicit dependency via global state:

• Assume:
  1. Module A writes some data D to global state
  2. Module B reads D from the global state
  3. Module B does not import module A. The dependency is implicit in reading D
  4. Module A is initialized prior to module B in the code
• Then:
  1. Initialization of A will fail due to the fact it writes to global state, hence it will be initialized only when the code is run
  2. Initialization of B succeeds during prepacking, but because A was not initialized, it might get different contents for D than it would have if A was initialized
  3. During runtime, A is initialized and writes to D correctly, but since B was already initialized it is oblivious to that value

To help detect such cases, we should emit a warning when a module fails to be prepacked because it writes to global state. Note that this won't guarantee that we catch all cases where a module initialization may affect global state (and hence can theoretically affect downstream modules), bit it should cover at least some of these cases.

[NTillmann]

I assume this is meant to address https://github.com/facebook/prepack/blob/master/src/serializer/modules.js#L90-L100?

[NTillmann]

Look out for #690 in the code to find the place where this needs to get addressed. #798 might be the key to make this safe, which is what should ultimately happen --- not just issuing a warning.