Remove Signal class and use Future/.whenComplete instead.

Remove Signal class and use Future/.whenComplete instead.

Committed: https://code.google.com/p/dart/source/detail?r=16792

[Lasse Reichstein Nielsen, 2013-01-08 08:38:27]

I recognized the same problem as dartbug.com/7762 though. Signal didn't need
error handling.

We can make whenComplete not tell the global error handler if an error isn't
handled (but then, I think we should do that everywhere anyway).

[floitsch, 2013-01-08 12:24:40]

LGTM.

https://codereview.chromium.org/11794043/diff/1/sdk/lib/async/future.dart
File sdk/lib/async/future.dart (right):

https://codereview.chromium.org/11794043/diff/1/sdk/lib/async/future.dart#new...
sdk/lib/async/future.dart:147: /** Supply a value for [future], or supply
[:null:] if a value is omitted. */
dartdoc comments should be third person:

/// Completes the [future] with the [value].
///
/// All subscriber of the [future] will be immediately informed of the
completion.

(or something similar).

I'm not sure we need to specify that the default value is `null`.