Implement Int64 as a wrapper around int

On AOT, JIT, and Wasm we don't need to emulate 64-bit integers as the
`int` type is already 64 bits.

This PR adds another implementation of `Int64` that is just a wrapper
around `int`:

    class Int64 implements IntX {
      final int _i;
      ...
    }

`int64` library imports this implementation in VM and dart2wasm, and the
old (emulated) `Int64` implementation on dart2js.

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

(In principle dart2wasm could support `dart:html` as well, but it won't
be supported, instead dart2wasm will support `package:web`)

Documentations of `Int64` and its members are copied directly from the
old emulated `Int64` class to make sure the documentation will look the
same as before.

The old `toRadixStringUnsigned` implementation is moved to the utilities
library and reused in the new class.

lib/fixnum.dart

@@ -5,7 +5,8 @@
 /// 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.
+/// identically whether executed on the Dart VM or compiled to JavaScript or
+/// Wasm.
 library fixnum;
 
 export 'src/int32.dart';