skip directives during nnbd migration

Currently, the migration engine crashes when it visits the identifier
in the following directive:

   import "dart:core" as one;

This updates the migration engine to skip directives when looking
for expressions and type annotations to be migrated.

Change-Id: I079263b13e5f4c75fbfe22fa06993e7a5d4f9c16
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/106660
Reviewed-by: Paul Berry <paulberry@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Dan Rubel <danrubel@google.com>

pkg/nnbd_migration/lib/src/graph_builder.dart

@@ -483,6 +483,12 @@ class GraphBuilder extends GeneralizingAstVisitor<DecoratedType> {
     return expressionType;
   }
 
+  @override
+  DecoratedType visitNamespaceDirective(NamespaceDirective node) {
+    // skip directives
+    return null;
+  }
+
   @override
   DecoratedType visitNode(AstNode node) {
     if (listener != null) {

pkg/nnbd_migration/test/graph_builder_test.dart

@@ -1435,6 +1435,15 @@ main() {
         hard: true);
   }
 
+  test_skipDirectives() async {
+    await analyze('''
+import "dart:core" as one;
+main() {}
+''');
+    // No test expectations.
+    // Just verifying that the test passes
+  }
+
   test_soft_edge_for_non_variable_reference() async {
     // Edges originating in things other than variable references should be
     // soft.