Support await keyword in cascade notation

[AAAbramenko]

Sometimes async methods of the class must be called sequentionally:

await loader.load('/file/path/1');
await loader.load('/file/path/2');

Support await in cascade notation will simplify this notation:

 loader
  ..await load('/file/path/1')
  ..await load('/file/path/2');

Awaited and unawated/sync calling methods can be mixed:

 loader
  ..setDestination('/destination/path') 
  ..await load('/file/path/1')
  ..await load('/file/path/2')
  ..removeFromSource(true);

[Cat-sushi]

this is related to #25 and #1216 .

[lrhn]

A suffix await seems like the best choice here.

The binding of prefix-cascade-await is iffy, and doesn't generalize to other chains.

loader..await load('file').save('file')

Does this await load('file') or does it await load('file').save('file').

And if save is also async, will you need ..await load('file').await save('file'), meaning this is not just about cascades.

Allowing loader..load('file').await.save('file').await will remove the ambiguity and follow the flow.

(It'd even allow a FutureOr<Null> foo = ...; bar(foo?.await); which doesn't await for null 😁)