[dart2wasm] Make int and double fields immutable

Make the `value` fields of `BoxedInt` and `BoxedDouble` `final`, to
generate immutable Wasm fields for them.

Immutable fields can potentially generate better code, as loads from the
same object will always generate the same value.

To allow making the `value` fields `final`, add a constructor.

To allow adding a constructor, "implement` base classes instead of
extending them. With `extends` the front-end wants us to call the
superclass constructors, even though they don't have any constructors.

This CL does not do the same to `BoxedBool`. We did that in a previous
iteration of the CL[1], but it caused regressions in post-TFA kernels.

[1]: https://dart-review.googlesource.com/c/sdk/+/393320

Change-Id: Ib44c25d09dcd84d999aa1d92dc9c38632454e443
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/393842
Commit-Queue: Ömer Ağacan <omersa@google.com>
Reviewed-by: Slava Egorov <vegorov@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>

pkg/dart2wasm/lib/class_info.dart

@@ -311,7 +311,9 @@ class ClassInfoCollector {
       // directly below the public classes they implement.
       // All other classes sit below their superclass.
       ClassInfo superInfo = cls == translator.coreTypes.boolClass ||
-              cls == translator.coreTypes.numClass
+              cls == translator.coreTypes.numClass ||
+              cls == translator.boxedIntClass ||
+              cls == translator.boxedDoubleClass
           ? topInfo
           : (!translator.options.jsCompatibility &&
                       cls == translator.wasmStringBaseClass) ||

sdk/lib/_internal/wasm/lib/boxed_double.dart

@@ -9,13 +9,13 @@ import 'dart:_string';
 import 'dart:_wasm';
 
 @pragma("wasm:entry-point")
-final class BoxedDouble extends double {
+final class BoxedDouble implements double {
   // A boxed double contains an unboxed double.
   @pragma("wasm:entry-point")
-  double value = 0.0;
+  final double value;
 
-  /// Dummy factory to silence error about missing superclass constructor.
-  external factory BoxedDouble();
+  @pragma("wasm:entry-point")
+  BoxedDouble._(this.value);
 
   static const int _mantissaBits = 52;
   static const int _exponentBits = 11;

sdk/lib/_internal/wasm/lib/boxed_int.dart

@@ -7,13 +7,13 @@ import 'dart:_internal';
 import 'dart:_wasm';
 
 @pragma("wasm:entry-point")
-final class BoxedInt extends int {
+final class BoxedInt implements int {
   // A boxed int contains an unboxed int.
   @pragma("wasm:entry-point")
-  int value = 0;
+  final int value;
 
-  /// Dummy factory to silence error about missing superclass constructor.
-  external factory BoxedInt();
+  @pragma("wasm:entry-point")
+  BoxedInt._(this.value);
 
   external num operator +(num other);
   external num operator -(num other);