Dart2JS/DDC: Re-use native BigInt where possible

[matanlurey]

V8/Blink release notes:
https://groups.google.com/forum/#!msg/v8-dev/x571Gr0khNo/y8Jk0_vSBAAJ

[piyushchauhan2011]

Any updates on this issue ?

[matanlurey]

Since this is not supported in all browsers, and works without re-using, it's not a priority.

[lexaknyazev]

Although Dart's BigInt works on its own, it is not directly interoperable with JS BigInt values that may come from external APIs linked via package:js.

[lexaknyazev]

An interop workaround:

import 'package:js/js.dart';
import 'package:js/js_util.dart';

@JS('BigInt')
external Function get _JSBigInt;

Object _toJsBigInt(BigInt value) => _JSBigInt(value.toString()) as Object;

BigInt _fromJsBigInt(Object jsBigInt) =>
    BigInt.parse(callMethod(jsBigInt, 'toString', const []) as String);

bool _isJsBigInt(Object jsBigInt) =>
    getProperty(getProperty(jsBigInt, 'constructor') as Object, 'name') ==
    'BigInt';