Add FutureGroup to dart:async

[munificent]

Web components has a nice FutureGroup class that lets you wait on a dynamically modifiable set of futures. It ends up coming in handy in a few places (like test.dart). Consider adding it to dart:core?

https://github.com/dart-lang/dart-web-components/blob/master/lib/src/utils.dart#L90

[justinfagnani]

I'm using web_ui's FutureGroup in buildtool. It's definitely handy, and we now have a suitable library to put it in in dart:async.

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cc @jmesserly.

[DartBot]

This comment was originally written by munificentbob...@gmail.com

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+1! We copy/pasted this into Pub to use it there too. Would be nice if it was just in dart:async.

[DartBot]

This comment was originally written by @seaneagan

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Often you have an Iterable<E> each of whose elements are mapped to a Future<T>, so it would be nice if it were FutureGroup<T>. And now that Future no longer exposes a "value" property, it would make more sense for FutureGroup.future to be a Future<List<T>> instead of a Future<List<Future<T>>>. Also, a FutureGroup.from constructor might be nice.

[jmesserly]

cc @sigmundch.
Set owner to @jmesserly.
Removed Type-Defect label.
Added Type-Enhancement label.
Changed the title to: "Add FutureGroup to dart:async".

[sigmundch]

We now have another copy in polymer :)

Florian/Lasse - what are your thoughts on adding this type to dart:async?

I'd be happy to create a patch for you, but I wanted to make sure you like the idea first.

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cc @jmesserly.
cc @floitschG.
cc @lrhn.
Removed the owner.

[munificent]

It's also in scheduled_test and test.py. :)

[floitschG]

I like it, but it should not be in dart:async, but in a package.

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Added NotPlanned label.

[justinfagnani]

It's already available in quiver.async: http://google.github.io/quiver-dart/docs/quiver.async/FutureGroup.html