Fix RegExp.exec (#194)

[ochafik]

• Limit transpilation to direct call on a regexp literal (otherwise, side-effects of multiple calls on a global regexp can't be easily replicated with Dart's RegExp; a quick visitation of the tree could tell us if a regexp is non-global or if exec is only called once on it, but that would be overkill so we're just backing out of x.exec(...) if x is not a /literal/)
• Optimize case where exec result is only index-accessed

[ochafik]

Now forbidding exec call on non-literals (since we can't easily replicate the multiple-calls-on-same-global-regexp case).

[mprobst]

LGTM.