repl: hide top-level await feature behind a flag
Refs: https://github.com/nodejs/node/pull/17807 Refs: https://github.com/nodejs/node/pull/15566#issuecomment-353428430
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
commit message follows commit guidelines