[vm,aot] Fix handling of exported extensions in TFA tree shaker

Fixes https://github.com/flutter/flutter/issues/42845

Change-Id: If4734c16a46f74547f9fc92e4d4e26a969a9f809
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/121861
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>

pkg/vm/lib/transformations/type_flow/transformer.dart

@@ -929,6 +929,8 @@ class _TreeShakerPass2 extends Transformer {
         return !shaker.isClassUsed(node);
       } else if (node is Typedef) {
         return !shaker.isTypedefUsed(node);
+      } else if (node is Extension) {
+        return !shaker.isExtensionUsed(node);
       } else {
         return !shaker.isMemberUsed(node as Member);
       }

tests/language_2/vm/regress_flutter_42845_lib.dart

@@ -0,0 +1,12 @@
+// Copyright (c) 2019, 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.
+
+extension TestExtension on int {
+  bool get isPositive => this > 0;
+  bool get isNegative => this < 0;
+}
+
+extension UnusedExtension on int {
+  bool get isReallyZero => this == 0;
+}

tests/language_2/vm/regress_flutter_42845_test.dart

@@ -0,0 +1,19 @@
+// Copyright (c) 2019, 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.
+
+// SharedOptions=--enable-experiment=extension-methods
+
+// Tests exported extensions.
+
+import "regress_flutter_42845_lib.dart";
+export "regress_flutter_42845_lib.dart" show TestExtension, UnusedExtension;
+
+import "package:expect/expect.dart";
+
+int i = 42;
+
+void main() {
+  Expect.isTrue(i.isPositive);
+  Expect.isFalse(i.isNegative);
+}