go/expect: account for the offset from the file.base for modfiles

Since parsing modfiles does not give us positions relative to the fileset, we need to account for this by adding the file's offset within the fileset to the positions of the markers within the file.

Change-Id: I826cec05cd77d37900d32bce2fb6e5edcbfea936
Reviewed-on: https://go-review.googlesource.com/c/tools/+/217341
Run-TryBot: Rohan Challa <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Rebecca Stambler <[email protected]>

Author: ridersofrohan

go/expect/extract.go

@@ -52,8 +52,18 @@ func Parse(fset *token.FileSet, filename string, content []byte) ([]*Note, error
 		if err != nil {
 			return nil, err
 		}
-		fset.AddFile(filename, -1, len(content)).SetLinesForContent(content)
-		return extractMod(fset, file)
+		f := fset.AddFile(filename, -1, len(content))
+		f.SetLinesForContent(content)
+		notes, err := extractMod(fset, file)
+		if err != nil {
+			return nil, err
+		}
+		// Since modfile.Parse does not return an *ast, we need to add the offset
+		// within the file's contents to the file's base relative to the fileset.
+		for _, note := range notes {
+			note.Pos += token.Pos(f.Base())
+		}
+		return notes, nil
 	}
 	return nil, nil
 }