The cascade operator ..

In Elixir we are use to apply multiple functions on a value with the pipe operator `|>`

```elixir
"Hello there"
|> String.upcase()
|> String.split()
```
The pipe operator takes the returned value of the function and apply the next function. This works nicely because of Elixir values are immutable and functions return a new value with the updated change from the functions.

Dart is object oriented. This means that the following doesn't work:

```dart
var l = [1,2,3];
l.add(4).add(5);
```
Here l is an list and the `add` function change the state of the list directly, it doesn't create a new list value. Moreover the [`add` function](https://api.dart.dev/stable/1.24.3/dart-core/List/add.html) as the following declaration:

```dart
void add(E value)
```
So `add` returns a `void` value and not the list itself.

One solution to add multiple values in a list could be:

```dart
void main(){
var a = [1,2,3];
a.add(4);
a.add(5);
print(a);
}
```

However the cascade operator `..` is here to make the code a be nicer.
The operator apply a method on the receiver/object, discard the returned value of that method and instead returns the receiver/object itself.

```dart
void main(){
var a = [1,2,3]
..add(4)
..add(5);
print(a);
}
```
ref:
- https://dart.dev/guides/language/language-tour#cascade-notation-
- https://news.dartlang.org/2012/02/method-cascades-in-dart-posted-by-gilad.html
- https://stackoverflow.com/questions/17025769/how-do-method-cascades-work-exactly-in-dart

No target file or test is named. Start by reading the Dart cascade-notation documentation and the linked references, then determine where this explanation belongs; done means the lesson explains the `..` operator with the shown list examples and accurate references.

Written by the indexing model from the issue text.