Implement Int64 as a wrapper for int when targeting native and Wasm

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

Becuase `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.

Author: osa1

pkgs/fixnum/lib/fixnum.dart

@@ -5,8 +5,9 @@
 /// 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.
-library;
+/// identically whether executed on the Dart VM or compiled to JavaScript or
+/// Wasm.
+library fixnum;
 
 export 'src/int32.dart';
 export 'src/int64.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) {