go/packages: remove .s files from go list's CompiledGoFiles

This is a workaround for a go list regression that broke
go/packages but went unnoticed by because of a missing
call to packages.PrintErrors, added here.

Updates golang/go#28749

Change-Id: I1819a6143134a422791106ac037d3458ef864322
Reviewed-on: https://go-review.googlesource.com/c/149237
Reviewed-by: Ian Cottrell <iancottrell@google.com>
diff --git a/go/packages/stdlib_test.go b/go/packages/stdlib_test.go
index ddecff1..7266524 100644
--- a/go/packages/stdlib_test.go
+++ b/go/packages/stdlib_test.go
@@ -35,6 +35,9 @@
 	if err != nil {
 		t.Fatalf("failed to load metadata: %v", err)
 	}
+	if packages.PrintErrors(pkgs) > 0 {
+		t.Fatal("there were errors loading standard library")
+	}
 
 	t1 := time.Now()
 	runtime.GC()
@@ -99,6 +102,10 @@
 			t.Errorf("Load failed: %v", err)
 			continue
 		}
+		if packages.PrintErrors(pkgs) > 0 {
+			t.Error("there were errors loading standard library")
+			continue
+		}
 		pkg := pkgs[0]
 		obj := pkg.Types.Scope().Lookup(test.name)
 		if obj == nil {