This can import all of the standard library, and has been tested
by using gotype to type check libgo with gccgo's export data (this
would be nice to automate, but I can't see a good way to do it,
not least because system-specific source files cause errors which
I needed to identify manually).
It includes a builtin export locator. Unfortunately I can't see a
more reliable way to locate the builtin export files than to parse
the output of 'gccgo -###'.
R=gri, iant, gri
CC=golang-codereviews, golang-dev
https://golang.org/cl/31860043
893253274d