Add convenience constructor: `Stream.single(T value)`

[tvolkert]

I often find myself returning streams with a single value. A convenience constructor would alleviate the following:

new Stream<T>.fromIterable(<T>[value])

[lrhn]

There is new Future<T>.value(value).asStream().

[tvolkert]

I lump that in with new Stream<T>.fromIterable(<T>[value]) -- both work well enough, but they obscure the code ever so slightly.

I'm of two minds on convenience methods/constructors. On one hand, they bloat the API, but on the other, they make calling code look cleaner...

[lrhn]

It is indeed a balancing problem. The utility constructor (or function or class) should be used often enough that it outweighs the extra complexity, be it space, namespace, or user consideration of whether to use it (which requires understanding it), that the feature adds just by existing.

I'm not sure a Stream.single carries its own weight in the platoform libraries, but it could if put in package:async (by being optional, not everybody is affected by the extra complexity, something the platform libraries cannot claim).