populateLines(fileContents) {
    this.instances.forEach(instance => {
      var lines = fileContents[instance.filename]
        .slice(instance.start.line - 1, instance.end.line)
        .join('\n');

      instance.lines = strip(lines);
    });
   }
Populates each match instance with a lines property containing the
relevant source code lines.

@param {object} fileContents The file paths and their contents