Play it safe with null tokens/nodes (#24910).

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 244 matching lines @@
 
   /// Return a visitor to be passed to pubspecs to perform lint
   /// analysis.
   /// Lint errors are reported via this [Linter]'s error [reporter].
   PubspecVisitor getPubspecVisitor() => null;
 
   @override
   AstVisitor getVisitor() => null;
 
   void reportLint(AstNode node) {
-    reporter.reportErrorForNode(new _LintCode(name, description), node, []);
+    if (node != null) {
+      reporter.reportErrorForNode(new _LintCode(name, description), node, []);
+    }
   }
 
   void reportLintForToken(Token token) {
-    reporter.reportErrorForToken(new _LintCode(name, description), token, []);
+    if (token != null) {
+      reporter.reportErrorForToken(new _LintCode(name, description), token, []);
+    }
   }
 
   void reportPubLint(PSNode node) {
     Source source = createSource(node.span.sourceUrl);
 
     // Cache error and location info for creating AnalysisErrorInfos
     // Note that error columns are 1-based
     var error = new AnalysisError(source, node.span.start.column + 1,
         node.span.length, new _LintCode(name, description));
 
@@ skipping 152 matching lines @@
 }
 
 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);
 }