anchor.diff:    Dart2JsSourceLocation(this._script, this._span);
   
     int _computeLine() {
  -    var sourceFile = _script.file as SourceFile;
  +    var sourceFile = _script.file;
       if (sourceFile != null) {
         return sourceFile.getLine(offset) + 1;
       }
  
positive.code: import '../resolution/resolution.dart' show Scope;
  import '../dart2jslib.dart';
  import '../dart_types.dart';
  import '../source_file.dart';
  import '../tree/tree.dart';
  import '../util/util.dart' show Spannable, Link;
  import '../util/characters.dart' show $CR, $LF;
  
negative.code:   int _computeColumn() {
      if (length == 0) return 0;
  
      var sourceFile = _script.file;
      if (sourceFile != null) {
        return sourceFile.getColumn(sourceFile.getLine(offset), offset) + 1;
      }
  