consider support conditional compile

such as support some macro to guide compiler generate different ast.
if cfg!(dart_version == "2.12") { // cfg! macro will generate concrete ast by dart_version
foo = 9;
} else {
foo = 10;
}
// use foo in following code

Start with the conditional-compilation proposal and cfg!(dart_version == "2.12") example in the issue. No file, test, or entry point is identified, so the syntax, AST-generation behavior, and version-selection semantics would need to be designed and documented before implementation can be considered done.

Written by the indexing model from the issue text.