repl: Failed to save editor mode text in `.save` command

Checklist

• make -j4 test (UNIX), or vcbuild test nosign (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

repl

Description of change

Fixes: https://github.com/nodejs/node/issues/8142

Note: This pull request depends on #8143 to merge in order to pass the test.