anchor.diff:  
   import 'dart:async';
   
  -import 'package:json/json.dart';
   
   import '../builder.dart';
   import '../jobs.dart';
   import '../workspace.dart';
   import '../package_mgmt/bower_properties.dart';
  +import 'json_parser.dart';
  +import 'utils.dart';
   
   /**
    * A [Builder] implementation to add validation warnings to JSON files.
  
positive.code: 
        try {
          if (str.trim().isNotEmpty) {
            JsonParser parser = new JsonParser(str, new _JsonParserListener(file));
            parser.parse();
          }
        } catch (e) {
  
negative.code: 
  class _JsonParserListener extends JsonListener {
    final File file;
    final StringLineOffsets lineOffsets;
  
    _JsonParserListener(this.file, this.lineOffsets);
  
    void fail(String source, Span span, String message) {
      int lineNum = lineOffsets.getLineColumn(span.start).line;
      file.createMarker('json', Marker.SEVERITY_ERROR, message, lineNum, span.start, span.end);
    }
  }
  