Fix cycle in override check.

pkg/analyzer/lib/src/generated/error_verifier.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.error_verifier;
 
 import 'dart:collection';
 import "dart:math" as math;
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 5784 matching lines @@
     DartType staticReturnType = getStaticType(returnExpression);
     if (staticReturnType != null && _enclosingFunction.isAsynchronous) {
       return _typeProvider.futureType.instantiate(
           <DartType>[staticReturnType.flattenFutures(_typeSystem)]);
     }
     return staticReturnType;
   }
 
   MethodElement _findOverriddenMemberThatMustCallSuper(MethodDeclaration node) {
     ExecutableElement overriddenMember = _getOverriddenMember(node.element);
-    while (overriddenMember is MethodElement) {
+    List<ExecutableElement> seen = <ExecutableElement>[];
+    while (
+        overriddenMember is MethodElement && !seen.contains(overriddenMember)) {
       for (ElementAnnotation annotation in overriddenMember.metadata) {
         if (annotation.isMustCallSuper) {
           return overriddenMember;
         }
       }
+      seen.add(overriddenMember);
       // Keep looking up the chain.
       overriddenMember = _getOverriddenMember(overriddenMember);
     }
     return null;
   }
 
   /**
    * Return the error code that should be used when the given class [element]
    * references itself directly.
    */
@@ skipping 446 matching lines @@
 class _InvocationCollector extends RecursiveAstVisitor {
   final List<String> superCalls = <String>[];
 
   @override
   visitMethodInvocation(MethodInvocation node) {
     if (node.target is SuperExpression) {
       superCalls.add(node.methodName.name);
     }
   }
 }