repl: treat "Assignment to const" as syntax error

Adding a new `repl-harmony` test file here because adding the
`--use_strict --harmony` flags on the main repl test file was causing
lots of unrelated failures, due to global variable assignments and
things like that. This new test file is based off of the original
repl.js test file, but has a lot of the tests stripped out. A test case
for this commit is included though.

Fixes #6132.