this also implements multitest support, which fixes#280
Fixes some other preexisting bugs:
* MetaLets did not simplify themselves correctly in some nested cases
* Library prefixed identifiers did not work as lvalues in opassign
* dsetindex/dput/[]= methods did not return a value
* checker did not correctly handle invalid constructor field initializers
* cascades did not correctly work with method invocations(?)
* postfix ++/-- did not correctly generate lvalues in some cases
The good news: because this reuses on our existing lvalue/metalet helpers, it managed to flush out a lot of bugs in other features that use them.
R=vsm@google.com
Review URL: https://codereview.chromium.org/1316723003 .
Leaf Petersen