[Feature Request] - Contracts

As it happens we write extension functions to check agains some validation, the compiler won't know that the value is validated inside a method as Not Null or something else and still will complain.

As example, on Kotlin there are Contracts.
https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.contracts/contract.html

On the code below, the compiler still will complain about Null check.

````dart
extension StringNullableExt on String? {
bool get isNullOrEmpty {
final instance = this;
return instance == null || instance.isEmpty;
}
}
````

````dart
String? somethingMayBeNull = arguments["something"];

if (somethingMayBeNull.isNullOrEmpty) return;

String notNull = somethingMayBeNull; // A value of type 'String?' can't be assigned to a variable of type 'String'.
````