Fix issue autocompleting members on dynamic values in ng plugin.

See angular issue #499 for a stack trace. When autocompleting a member
on dynamic, the member contributor checks to find a member definition
higher up in the tree that should lead to specific members to suggest.

However, in the angular plugin, the ASTs passed in are often fragments.
Account for the case where a node has no parent earlier than expected.

Change-Id: Ib30d9f1aef77241224f604c5994489aa9448b0c2
Reviewed-on: https://dart-review.googlesource.com/42884
Commit-Queue: Mike Fairhurst <mfairhurst@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

pkg/analyzer_plugin/lib/src/utilities/visitors/local_declaration_visitor.dart

@@ -186,7 +186,9 @@ abstract class LocalDeclarationVisitor extends GeneralizingAstVisitor {
 
   @override
   void visitNode(AstNode node) {
-    node.parent.accept(this);
+    // Support the case of searching partial ASTs by aborting on nodes with no
+    // parents. This is useful for the angular plugin.
+    node.parent?.accept(this);
   }
 
   @override