Source maps should only be activated via the `--map` flag.
Fix line numbering for invoke instructions.

Line numbering of exceptions now work (better) in optimized mode.
Make test_debug actually fail when things go wrong.
Make optimizer handle both strings and string-like type objects.

NodeWithToken is a string-like type produced by the parser during 'embed
tokens' mode, which allows us to track line numbers.
Put uglify back the way it was.

Makes for an easier-to-review pull request.
Improvements to test_html_source_map.

* Now works on FF Nightly
* Test does not run unless explicitly requested
Get test_source_map passing again.
Test that source mapping works with EMCC_DEBUG=2.
Ensure JS output remains the same when making source maps.
Centralize debug options in a -gX flag.
Fix up the source map JS output 'reftest'.

Without re.MULTILINE, the regexp wasn't actually matching anything...