The interpreter's os.Exit now triggers a special panic rather
than kill the test process. (It's semantically dubious, since
it will run deferred routines.) Interpret now returns its
exit code rather than calling os.Exit.
Also:
- disabled parts of a few $GOROOT/tests via os.Getenv("GOSSAINTERP").
- remove unnecessary 'slots' param to external functions; they
are never closures.
Most of the tests are disabled until go/types supports shifts.
They can be reenabled if you patch this workaround:
https://golang.org/cl/7312068
R=iant, bradfitz
CC=golang-dev, gri
https://golang.org/cl/7313062