AbiSpecificInteger class Null safety

The supertype of all Abi-specific integer types.

Abi-specific integers should extend this class and annotate it with AbiSpecificIntegerMapping to declare the integer size and signedness for Abi.values.

For example:

/// Represents a `uintptr_t` in C.
///
/// [UintPtr] is not constructible in the Dart code and serves purely as
/// marker in type signatures.
@AbiSpecificIntegerMapping({
  Abi.androidArm: Uint32(),
  Abi.androidArm64: Uint64(),
  Abi.androidIA32: Uint32(),
  Abi.androidX64: Uint64(),
  Abi.fuchsiaArm64: Uint64(),
  Abi.fuchsiaX64: Uint64(),
  Abi.iosArm: Uint32(),
  Abi.iosArm64: Uint64(),
  Abi.linuxArm: Uint32(),
  Abi.linuxArm64: Uint64(),
  Abi.linuxIA32: Uint32(),
  Abi.linuxX64: Uint64(),
  Abi.macosArm64: Uint64(),
  Abi.macosX64: Uint64(),
  Abi.windowsIA32: Uint32(),
  Abi.windowsX64: Uint64(),
})
class UintPtr extends AbiSpecificInteger {
  const UintPtr();
}