Add a utility method so that it can be added to the superclass without breaking the build

lib/src/linter.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library linter.src.linter;
 
 import 'dart:io';
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/src/generated/engine.dart'
@@ skipping 407 matching lines @@
           contents: sourceFile.readAsStringSync(), sourcePath: sourceFile.path);
 }
 
 class _LineInfo implements LineInfo {
   PSNode node;
   _LineInfo(this.node);
 
   @override
   LineInfo_Location getLocation(int offset) => new LineInfo_Location(
       node.span.start.line + 1, node.span.start.column + 1);
+
+  int getOffsetOfLine(int lineNumber) {
+    throw new UnsupportedError('Cannot get line offset from a yaml node');
+  }
 }
 
 class _LintCode extends LintCode {
   static final registry = <String, LintCode>{};
 
   factory _LintCode(String name, String message) => registry.putIfAbsent(
       name + message, () => new _LintCode._(name, message));
 
   _LintCode._(String name, String message) : super(name, message);
 }