[dart2wasm] Ensure the field in BoxedBool is non mutable

Change-Id: Idfa251e647f9d0205086d17334103a511366df7a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/501460
Commit-Queue: Martin Kustermann <kustermann@google.com>
Reviewed-by: Slava Egorov <vegorov@google.com>

pkg/dart2wasm/test/ir_tests/dynamic_call.wat

@@ -4,7 +4,7 @@
   (type $Array<Object?> (array (field (mut (ref null $#Top)))))
   (type $BoxedBool (sub final $#Top (struct
     (field $field0 i32)
-    (field $value (mut i32)))))
+    (field $value i32))))
   (type $BoxedInt (sub final $#Top (struct
     (field $field0 i32)
     (field $value i64))))

sdk/lib/_internal/wasm/lib/boxed_bool.dart

@@ -2,14 +2,15 @@
 // 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.
 
+import 'dart:_wasm';
+
 @pragma("wasm:entry-point")
 final class BoxedBool extends bool {
-  // A boxed bool contains an unboxed bool.
   @pragma("wasm:entry-point")
-  bool value = false;
+  final WasmI32 value;
 
-  /// Dummy factory to silence error about missing superclass constructor.
-  external factory BoxedBool();
+  @pragma("wasm:entry-point")
+  external BoxedBool();
 
   @override
   bool operator ==(Object other) {