Go1.10 now modifies CGo sources as text, not AST, and this way comments
are not removed and gotrace can see its '//trace:event ' comments in a
CGo *.go files: https://github.com/golang/go/commit/85c3ebf4.

This way with Go1.10 tests were failing because `//trace:event
traceHello()` in a/pkg1/pkg1c.go was now noticed by gotrace and more
then expected trace events produces.

I have not noticed this in 65c399f0 (tracing/runtime: Add support for
Go1.10 (preliminary)) probably because at that time above Go commit was
not in my local Go1.10 tree.

For the reference: tracing/runtime support regenerated for Go1.10.2
stays exactly the same as in 65c399f0 and so does not need updating.