Implement Int64 as a wrapper for int when targeting native and Wasm (#905)

This PR adds another `Int64` implementation that is just a wrapper around
`int`, to be used in targets where `int` is 64 bits.

Because `dart doc` generates using the native class, all documentation comments
are moved to the native class.

The conditional import is implemented based on availability of the library
`dart:html`, which is only available on dart2js.

Improves #172. It could be further improved by having `Int64` as an extension
class when targeting native and Wasm, but that would be a breaking change, and
we would have to remove the `IntX` superclass as extensions cannot extend or
implement other classes.

Author: osa1

pkgs/fixnum/CHANGELOG.md

@@ -5,6 +5,7 @@
   like `sortedBy` in the platform libraries.
 * Run `dart format` with the new style.
 * Require Dart 3.4
+* Improve `Int64` representation when compiling to native or Wasm.
 
 ## 1.1.1
 

pkgs/fixnum/lib/fixnum.dart

@@ -4,8 +4,8 @@
 
 /// Signed 32- and 64-bit integer support.
 ///
-/// The integer implementations in this library are designed to work
-/// identically whether executed on the Dart VM or compiled to JavaScript.
+/// The integer implementations in this library are designed to work identically
+/// whether executed on the Dart VM or compiled to JavaScript or Wasm.
 library;
 
 export 'src/int32.dart';

pkgs/fixnum/lib/src/int32.dart

@@ -321,8 +321,6 @@ class Int32 implements IntX {
     return Int32(value);
   }
 
-  /// Returns [:true:] if this [Int32] has the same numeric value as the
-  /// given object.  The argument may be an [int] or an [IntX].
   @override
   bool operator ==(Object other) {
     if (other is Int32) {
@@ -437,10 +435,10 @@ class Int32 implements IntX {
   @override
   List<int> toBytes() {
     var result = List<int>.filled(4, 0);
-    result[0] = _i & 0xff;
-    result[1] = (_i >> 8) & 0xff;
-    result[2] = (_i >> 16) & 0xff;
     result[3] = (_i >> 24) & 0xff;
+    result[2] = (_i >> 16) & 0xff;
+    result[1] = (_i >> 8) & 0xff;
+    result[0] = _i & 0xff;
     return result;
   }