Go strong mode!

0.22.0 - 2015-04-21

BREAKING CHANGE: streams and async libraries have been merged into one async library
BREAKING CHANGE: Pre-1.8.0 SDKs are no longer supported.
Quiver is now strong mode compliant
New: Optional now implements Iterable and its methods are generic (using temporary syntax)
New: isNotEmpty and isDigit in strings.dart
New: Multimap.fromIterable
Fix: Change TreeSearch from class to enum.
Fix: fake_async.dart timers are now active while executing the callback