fix #25171, clear "hasBeenInferred" flag when re-running resolution

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/src/generated/ast.dart';
 import 'package:analyzer/src/generated/constant.dart';
@@ skipping 11727 matching lines @@
     }
     return null;
   }
 
   @override
   Object visitClassDeclaration(ClassDeclaration node) {
     _hasReferenceToSuper = false;
     super.visitClassDeclaration(node);
     ClassElementImpl classElement = _getClassElement(node.name);
     if (classElement != null) {
+      // Clear this flag, as we just invalidated any inferred member types.

[Leaf, 2015/12/11 00:09:25]

We've also invalidated anything that extends/implements/mixes this class. 
Presumably that is captured by the task model - if we're re-resolving a unit,
then we will be re-resolving everything in it's library cycle, and everything
that depends on it?

[Jennifer Messerly, 2015/12/11 00:15:51]

The invariant here is we'll clear the flag at basically the same time we visit
and clear the methods. So the order is:

superclass update method & superclass clear flag
...
class update method & class clear flag


In other words, because we are only operating on one class at a time in this
visitor, we shouldn't need to worry about other classes/libraries/task
model/whatever. 

If I understand the code correctly. :)

+      classElement.hasBeenInferred = false;
       classElement.hasReferenceToSuper = _hasReferenceToSuper;
     }
     return null;
   }
 
   @override
   void visitClassDeclarationInScope(ClassDeclaration node) {
     super.visitClassDeclarationInScope(node);
     ExtendsClause extendsClause = node.extendsClause;
     WithClause withClause = node.withClause;
@@ skipping 1560 matching lines @@
     nonFields.add(node);
     return null;
   }
 
   @override
   Object visitNode(AstNode node) => node.accept(TypeResolverVisitor_this);
 
   @override
   Object visitWithClause(WithClause node) => null;
 }