Fix incorrect instance checks.

Several instance checks added during the null safety core library
migration were incorrectly rejecting null values in places where null
values can still flow in legacy mode.  This eliminates the instance
checks in favor of casts which will behave as desired.

Bug: #41465
Change-Id: I50fb9e6cef41d645d5312871932c7168f2d80c29
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/143333
Commit-Queue: Leaf Petersen <[email protected]>
Reviewed-by: Nate Bosch <[email protected]>

Author: leafpetersen

sdk_nnbd/lib/async/future.dart

@@ -223,9 +223,8 @@ abstract class Future<T> {
       if (result is Future<T>) {
         return result;
       } else {
-        if (result is! T)
-          throw "unreachable"; // TODO(lrn): Remove when type promotion works.
-        return new _Future<T>.value(result);
+        // TODO(40014): Remove cast when type promotion works.
+        return new _Future<T>.value(result as dynamic);
       }
     } catch (error, stackTrace) {
       var future = new _Future<T>();

sdk_nnbd/lib/async/stream.dart

@@ -523,10 +523,8 @@ abstract class Stream<T> {
           subscription.pause();
           newValue.then(add, onError: addError).whenComplete(resume);
         } else {
-          if (newValue is! E) {
-            throw "unreachable"; // TODO(lrn): Remove when type promotion works.
-          }
-          controller.add(newValue);
+          // TODO(40014): Remove cast when type promotion works.
+          controller.add(newValue as dynamic);
         }
       });
       controller.onCancel = subscription.cancel;

tests/corelib/weak_mode/README

@@ -0,0 +1,2 @@
+This directory contains tests specific to null safety weak mode
+behavior.

tests/corelib/weak_mode/asyncMap_accepts_null.dart

@@ -0,0 +1,17 @@
+// 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.
+
+// @dart = 2.7
+
+import "dart:async";
+import "package:expect/expect.dart";
+
+// Regression test for https://github.com/dart-lang/sdk/issues/41465
+
+void main() {
+  var stream = Stream.fromIterable([1, 2, 3, 4]);
+  var eventsStream =
+      stream.asyncMap((i) => i % 2 == 0 ? i : null).where((i) => i != null);
+  eventsStream.listen(Expect.isNotNull);
+}

tests/corelib/weak_mode/future_sync_accepts_null.dart

@@ -0,0 +1,15 @@
+// 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.
+
+// @dart = 2.7
+
+import "dart:async";
+import "package:expect/expect.dart";
+
+// Regression test for https://github.com/dart-lang/sdk/issues/41465
+
+void main() async {
+  var f = Future<int>.sync(() => null);
+  Expect.isNull(await f);
+}