Use Dart's native class for big numbers instead of custom Uint64

[fajarvm]

Currently, this project is using its own class Uint64 to deal with big numbers. Dart's core lib has a BigInt class that can be used to work with big numbers.

However, there is currently an issue withBigInt when compiled to JavaScript (via dart2js), integers are therefore restricted to 53 significant bits because all JavaScript numbers are double-precision floating point values.

Please refer to this Dart language documentation page for information regarding big numbers and their known issues.
See also: https://github.com/dart-lang/sdk/blob/master/docs/language/informal/int64.md

When Dart SDK is updated with a BigInt class (or whichever replaces it) that is compatible with dart2js, this project should also be updated to use that class instead of Uint64.