Correcting class inheritance and a few bugs I encountered

[vrescobar]

Hi, as mentioned I am correcting the issue #753 and a few things I might find while I try to use transcrypt for my webpage. Hope you can come back soon to review the changes and make them mainstream.

P.S. I might do some other PRs to the npm webpack branch

closes #753

[vrescobar]

I am also dealing with a few other bugs (sometimes the compiler tried to parse a non existing path and ended up crashing with double exceptions).

Please see the details on each commit and the issue each one of them it closes