[ffigen] Generate assert or comment about the package:objective_c version

[liamappelbe]

FFIgen and objective_c are pretty tightly coupled. It might make sense to code gen a comment or assert about the version of package:objective_c that the generated code expects.

// WARNING: This generated code uses package:objective_c v9.2.0
import 'package:objective_c/objective_c.dart' as objc;

void _versionCheck() {  // We'd need to call this somewhere in the bindings. Maybe when loading class objects?
  assert(objc.version.major == 9 && objc.version.minor >= 2); 
}

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A more out-there approach would be something like this, which enforces the tight coupling at compile time:

// In FFIgen's generated code... 

import 'package:objective_c/objective_c.dart' as objc;

const _requireObjectiveCVersion = objc.version_9_2_0;

To enforce that the major versions are equal, and that package:objective_c's minor version is greater than or equal to the one in the generated code, package:objective_c would define these version getters for each minor version in the current major version:

const bool version_9_0_0 = true;
const bool version_9_1_0 = true;
const bool version_9_2_0 = true;
const bool version_9_3_0 = true;

We'd probably want to codegen these, and have a test to make sure they're in sync with the pubspec version.