[dart2js] Test for _Required sentinel in deferred apply

Change-Id: I962ec623b93549fe5a395ab086aa04dfee7f144e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/151140
Commit-Queue: Stephen Adams <sra@google.com>
Reviewed-by: Mark Zhou <markzipan@google.com>

tests/dart2js/deferred/deferred_apply_lib.dart

@@ -0,0 +1,13 @@
+// Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library deferred_apply_lib;
+
+String apply1(Function f) {
+  return Function.apply(f, [], {#req1: 10});
+}
+
+String apply2(Function f) {
+  return Function.apply(f, []);
+}

tests/dart2js/deferred/deferred_apply_test.dart

@@ -0,0 +1,69 @@
+// Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Requirements=nnbd-strong
+
+import 'package:expect/expect.dart';
+import 'deferred_apply_lib.dart' deferred as lib;
+
+// Test that the `_Required` sentinel in apply-metadata is initialized.
+//
+// This test is structured so that `Function.apply` is partioned out of the main
+// unit into a 'part' file. `Function.apply` contains a reference to the
+// `_Required` sentinel marker, which causes the marker to also be in that part.
+// The apply-metadata for the functions [foo1] and [foo2] contains references to
+// the sentinel that are in the main unit.
+//
+// This apparent reference to the deferred-loaded constant is not a problem
+// because the apply-metadata is contained in a JavaScript function which is not
+// called until the function is passed to `Function.apply`. The loading of the
+// part containing `Function.apply` also loads the sentinel in advance of it
+// being referenced.
+//
+// An implicit constraint here is that the apply-metadata must not be used
+// outside of code that also references the sentinel.
+//
+// If the apply-metadata is created eagerly, or we add other uses of the
+// apply-metadata, then we would need to ensure the constant is located to be
+// available at that time.
+
+main() {
+  lib.loadLibrary().then((_) {
+    Expect.equals('10', lib.apply1(foo1));
+    Expect.equals('10,$text1', lib.apply1(foo2));
+    Expect.throws(() => lib.apply2(foo2));
+  });
+}
+
+class Text {
+  final String string;
+  final String source;
+  const Text(this.string, [this.source = '']);
+  String toString() => string;
+}
+
+String foo1({required int req1}) {
+  return '$req1';
+}
+
+String foo2({required int req1, Text? text = text1}) {
+  return '$req1,$text';
+}
+
+const text1 = Text(r"""
+Those lines that I before have writ do lie,
+Even those that said I could not love you dearer:
+Yet then my judgment knew no reason why
+My most full flame should afterwards burn clearer.
+But reckoning Time, whose million'd accidents
+Creep in 'twixt vows, and change decrees of kings,
+Tan sacred beauty, blunt the sharp'st intents,
+Divert strong minds to the course of altering things;
+Alas! why, fearing of Time's tyranny,
+Might I not then say, 'Now I love you best,'
+When I was certain o'er incertainty,
+Crowning the present, doubting of the rest?
+   Love is a babe, then might I not say so,
+   To give full growth to that which still doth grow?
+""");