go/types: add -halt flag to ease debugging in test mode

Specifying -halt in `go test -run Check$ -halt` causes a panic
upon encountering the first error. The stack trace is useful to
determine what code path issued the error.

Change-Id: I2e17e0014ba87505b01786980b98565f468065bf
Reviewed-on: https://go-review.googlesource.com/c/go/+/190257Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/go/types/check_test.go

@@ -42,8 +42,9 @@ import (
 )
 
 var (
-	listErrors = flag.Bool("errlist", false, "list errors")
-	testFiles  = flag.String("files", "", "space-separated list of test files")
+	haltOnError = flag.Bool("halt", false, "halt on error")
+	listErrors  = flag.Bool("errlist", false, "list errors")
+	testFiles   = flag.String("files", "", "space-separated list of test files")
 )
 
 // The test filenames do not end in .go so that they are invisible
@@ -262,6 +263,9 @@ func checkFiles(t *testing.T, testfiles []string) {
 	}
 	conf.Importer = importer.Default()
 	conf.Error = func(err error) {
+		if *haltOnError {
+			defer panic(err)
+		}
 		if *listErrors {
 			t.Error(err)
 			return