Guard against NPE

pkg/analyzer/lib/src/task/strong_mode.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 analyzer.src.task.strong_mode;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
@@ skipping 261 matching lines @@
     // we must express its parameter and return types in terms of its own
     // parameters. For example, given `m<T>(t)` overriding `m<S>(S s)` we
     // should infer this as `m<T>(T t)`.
     //
     List<DartType> typeFormals =
         TypeParameterTypeImpl.getTypes(element.type.typeFormals);
 
     List<FunctionType> overriddenTypes = new List<FunctionType>();
     for (ExecutableElement overriddenMethod in overriddenMethods) {
       FunctionType overriddenType = overriddenMethod.type;
+      if (overriddenType == null) {
+        // TODO(brianwilkerson) I think the overridden method should always have
+        // a type, but there appears to be a bug that causes it to sometimes be
+        // null, we guard against that case by not performing inference.
+        return;
+      }
       if (overriddenType.typeFormals.isNotEmpty) {
         if (overriddenType.typeFormals.length != typeFormals.length) {
           return;
         }
         overriddenType = overriddenType.instantiate(typeFormals);
       }
       overriddenTypes.add(overriddenType);
     }
 
     //
@@ skipping 167 matching lines @@
 
   @override
   void visitSimpleIdentifier(SimpleIdentifier node) {
     if (!node.inDeclarationContext()) {
       Element nonAccessor(Element element) {
         if (element is PropertyAccessorElement && element.isSynthetic) {
           return element.variable;
         }
         return element;
       }
+
       Element element = nonAccessor(node.staticElement);
       if (element is VariableElement && (filter == null || filter(element))) {
         results.add(element);
       }
     }
   }
 }
 
 /**
  * A class of exception that is not used anywhere else.
  */
 class _CycleException implements Exception {}