Migration: Handle uses of typedef types as expressions.

Addresses ~44 exceptions whose stack trace contains the line:

EdgeBuilder.visitSimpleIdentifier (package:nnbd_migration/src/edge_builder.dart:1123:7)

Change-Id: I5c76b7478d15d2338d7b47cbc6bebcf8c0d2d739
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/116245
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

pkg/nnbd_migration/lib/src/edge_builder.dart

@@ -1116,7 +1116,7 @@ class EdgeBuilder extends GeneralizingAstVisitor<DecoratedType>
       return staticElement.isGetter
           ? elementType.returnType
           : elementType.positionalParameters[0];
-    } else if (staticElement is ClassElement) {
+    } else if (staticElement is TypeDefiningElement) {
       return _nonNullableTypeType;
     } else {
       // TODO(paulberry)

pkg/nnbd_migration/test/edge_builder_test.dart

@@ -4505,7 +4505,15 @@ void f(List<int> x) {}
         hard: true);
   }
 
-  test_typeName() async {
+  test_typeName_class() async {
+    await analyze('''
+class C {}
+Type f() => C;
+''');
+    assertNoUpstreamNullability(decoratedTypeAnnotation('Type').node);
+  }
+
+  test_typeName_from_sdk() async {
     await analyze('''
 Type f() {
   return int;
@@ -4514,14 +4522,38 @@ Type f() {
     assertNoUpstreamNullability(decoratedTypeAnnotation('Type').node);
   }
 
-  test_typeName_prefixed() async {
+  test_typeName_from_sdk_prefixed() async {
     await analyze('''
 import 'dart:async' as a;
 Type f() => a.Future;
 ''');
     assertEdge(never, decoratedTypeAnnotation('Type').node, hard: false);
   }
 
+  test_typeName_functionTypeAlias() async {
+    await analyze('''
+typedef void F();
+Type f() => F;
+''');
+    assertNoUpstreamNullability(decoratedTypeAnnotation('Type').node);
+  }
+
+  test_typeName_genericTypeAlias() async {
+    await analyze('''
+typedef F = void Function();
+Type f() => F;
+''');
+    assertNoUpstreamNullability(decoratedTypeAnnotation('Type').node);
+  }
+
+  test_typeName_mixin() async {
+    await analyze('''
+mixin M {}
+Type f() => M;
+''');
+    assertNoUpstreamNullability(decoratedTypeAnnotation('Type').node);
+  }
+
   test_typeName_union_with_bound() async {
     await analyze('''
 class C<T extends Object> {}