Lowering the requirements for default const parameters

Documentation says that

> The default values must be compile-time constants

I'm curious why `const` values are required. I think it would be better if this requirement will be lowering a bit. It'll be awesome to be able to use **previous passed arguments** as default parameters or every time create a new **modifiable** default list in place of using `const` one:
```dart
void read(byte[] bytes, {int offset = 0, int length = bytes.size, byte[] avoid = []}) {
...
}
```