--inspect-brk does not work with ESM

[TimothyGu]

• Version: master
• Platform: Linux 4.9.0-4-amd64 SMP Debian 4.9.51-1 (2017-09-28) x86_64 GNU/Linux
• Subsystem: esm, inspector

From #17336.

$ echo "console.log('abc');" > a.mjs
$ node --inspect-brk --experimental-modules a.mjs
Debugger listening on ws://127.0.0.1:9229/bd0babdd-f59c-4014-8fcf-5679cdbcdc83
For help see https://nodejs.org/en/docs/inspector
# Connect to node instance with Chrome DevTools. Then, without doing anything
# in DevTools...
Debugger attached.
(node:2995) ExperimentalWarning: The ESM module loader is experimental.
abc
Waiting for the debugger to disconnect...