Fix for recording 'null' as a defined element.

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 1c3a644d230aa3c51518fe6a85c617a7e7cfd14f..ac5513cc217f76725aa36f76daa37d143f8b9a04 100644
--- a/pkg/analyzer/lib/src/generated/resolver.dart
+++ b/pkg/analyzer/lib/src/generated/resolver.dart
@@ -4073,7 +4073,9 @@ class GatherUsedLocalElementsVisitor extends RecursiveAstVisitor {
   visitSimpleIdentifier(SimpleIdentifier node) {
     Element element = node.staticElement;
     if (node.inDeclarationContext()) {
-      definedElements.add(element);
+      if (element != null) {
+        definedElements.add(element);
+      }
       return;
     }
     bool isIdentifierRead = _isReadIdentifier(node);