Fix for recording 'null' as a defined element.

pkg/analyzer/lib/src/generated/resolver.dart

 // Copyright (c) 2014, 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 analyzer.src.generated.resolver;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
@@ skipping 4055 matching lines @@
       super.visitMethodDeclaration(node);
     } finally {
       _enclosingExec = enclosingExecOld;
     }
   }
 
   @override
   visitSimpleIdentifier(SimpleIdentifier node) {
     Element element = node.staticElement;
     if (node.inDeclarationContext()) {
-      definedElements.add(element);
+      if (element != null) {
+        definedElements.add(element);
+      }
       return;
     }
     bool isIdentifierRead = _isReadIdentifier(node);
     if (element is LocalVariableElement) {
       if (isIdentifierRead) {
         usedElements.addElement(element);
       }
     } else {
       _useIdentifierElement(node);
       if (element == null ||
@@ skipping 8595 matching lines @@
     nonFields.add(node);
     return null;
   }
 
   @override
   Object visitNode(AstNode node) => node.accept(TypeResolverVisitor_this);
 
   @override
   Object visitWithClause(WithClause node) => null;
 }