Default argument values should imply "const" context

If you run this:

```
class Test {
Test({ this.test = Duration(minutes: 1) }); // the default value is not explicitly "const"
final Duration test;
}

void main() {
Test();
}
```

...you get an exception (a bad one, currently, see https://github.com/dart-lang/sdk/issues/35683).

I think the fact that only a constant is allowed to be used there should imply a constant context and thus make the "const" redundant and implied.