Several Flutter tools doesn't use Dart language versioning #70084
Description
Discovered while doing #70078: The logic in flutter updates-packages calculates package versions by writing a temporary pubspec.yaml file. This file doesn't have an SDK constraint, so it's language version is undefined.
/// Generates the source of a fake pubspec.yaml file given a list of
/// dependencies.
String _generateFakePubspec(Iterable<PubspecDependency> dependencies) {
final StringBuffer result = StringBuffer();
final StringBuffer overrides = StringBuffer();
result.writeln('name: flutter_update_packages');
result.writeln('dependencies:');
overrides.writeln('dependency_overrides:');