Null message error on build-time assert

[cmc5788]

At runtime, including either an implicit or an explicit null assert message seems to behave correctly. In other words, both of these work as expected:

assert(false, null); // Failed assertion: ... 'false': is not true.
assert(false); // Failed assertion: ... 'false': is not true.

However, at build-time, asserts with an explicit null message will fail. I discovered this when I was building the following micro-library intended to make it easier for us to use build-time asserts;

https://github.com/cmc5788/global_assert#why-not-allow-an-assert-without-a-message
https://pub.dev/packages/global_assert

A minimal reproduction sample is as follows;

class BuildAssert {
  const BuildAssert(bool condition, [String? message])
      : assert(condition, message);
}

const _assert = BuildAssert(false);

void main() {}

Compiling this will fail with:

Context: Expected constant 'null' to be of type 'String', but was of type 'Null'.

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• Dart SDK Version (dart --version)

Dart SDK version: 2.15.0-268.18.beta (beta) (Wed Nov 17 11:39:23 2021 +0100) on "macos_x64"