Frog generates invalid JS with an assert() as the else clause

[munificent]

Compile this code with frog:

main() {
  if (true) print('ok'); else assert('unreachable');
}

And it generates:

function main() {
  if (true) print$("ok");
  else // :(
}

[anders-sandholm]

Removed Area-Frog label.
Added Area-Dart2JS, FromAreaFrog labels.

[kasperl]

Added WontFix label.