cmd/compile: if -G=3 fully enabled in compiler, go test cmd/go -test.run TestScript/mod_edit_go fails

[danscales]

For current master, if you enable -G=3 in the compiler by adding 'Flag.G = 3' at the end of ParseFlags in cmd/compile/internal/base/flag.go, then this command fails

go test cmd/go -test.run TestScript/mod_edit_go

with error:

--- FAIL: TestScript (0.01s)
    --- FAIL: TestScript/mod_edit_go (0.03s)
        script_test.go:254: 
            # Test support for go mod -edit to set language version. (0.017s)
            > env GO111MODULE=on
            > ! go build
            [stderr]
            # m
            ./alias.go:2:6: type aliases requires go1.9 or later
            [exit status 2]
            > stderr 'type aliases only supported as of'
            FAIL: testdata/script/mod_edit_go.txt:5: no match for `(?m)type aliases only supported as of` found in stderr

So, as we can see, the types2 typechecker is printing 'type aliases requires go1.9 or later', whereas our current noder.go prints 'type aliases only supported as of -lang=go1.9'.

We should probably reconcile these two so that the same test works for both, and we can get through the gotests with -G=3 fully enabled (i.e. using the types2 typechecker) without failing.

[danscales]

@griesemer

[gopherbot]

Change https://golang.org/cl/314776 mentions this issue: cmd/compile/internal/types2: match compiler error for invalid type alias decl