Fix for FormalParameter without name.

R=brianwilkerson@google.com

Change-Id: I9dd957a9a55aef444f1e82a094dbf600a57074b0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/99732
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

Author: scheglov

pkg/analyzer/lib/src/summary2/linked_unit_context.dart

@@ -431,7 +431,7 @@ class LinkedUnitContext {
     } else if (node is EnumConstantDeclaration) {
       return node.name.offset;
     } else if (node is FormalParameter) {
-      return node.identifier.offset;
+      return node.identifier?.offset ?? -1;
     } else if (node is MethodDeclaration) {
       return node.name.offset;
     } else if (node is NamedCompilationUnitMember) {