VM crash on await in assert.

[lrhn]

Example code:
  main() async {
    assert(await false);
    print("Didn't throw");
  }

When run in debug build of VM, ends with:

runtime/vm/flow_graph_builder.cc:2314: error: unreachable code
Aborted (core dumped)

[iposva-google]

cc @mraleph.
cc @fsc8000.
cc @sgmitrovic.
Set owner to @crelier.
Added Accepted label.

[crelier]

Fixed at r44388.

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Added Fixed label.