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

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

Index: pkg/analyzer/lib/src/generated/resolver.dart
diff --git a/pkg/analyzer/lib/src/generated/resolver.dart b/pkg/analyzer/lib/src/generated/resolver.dart
index a32a518326ccc19272b511eece4494ed778cacf9..03452f455ec2b47fcbcc0f9c158324328b71f2b8 100644
--- a/pkg/analyzer/lib/src/generated/resolver.dart
+++ b/pkg/analyzer/lib/src/generated/resolver.dart
@@ -11745,6 +11745,8 @@ class TypeResolverVisitor extends ScopedVisitor {
     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;