Unlike JVM languages like Java and Kotlin, Dart doesn’t compile to Java bytecode. This makes it unsuitable for making extensions. Moreover, it’s not an officially supported language for native Android development, so, first-class for it isn’t possible at the moment.
Although, there exist some ways which makes Dart/Flutter interop possible in existing Android applications. This involves converting the required Dart/Flutter code to an AAR library and using it like a normal dependency.
But given the current state of AAR support for extensions, the above method is of no use for making extensions
Read the following topic. It has everything you need to get started with Rush.