Support runBlocking with arbitrary context

The current behavior of runBlocking with a specified context is somewhat low-level and hardly used in practice. In particular, unless the dispatcher that is passed to runBlocking implements EventLoop (usually - not), then runBlocking creates its own private event loop and uses it as dispatcher, while ignoring the dispatcher that was passed to it in the context.

This proposal is to change this behavior so that runBlocking(UI) { ... } would result in blocking the current thread and running the code in the context of UI dispatcher, consistently with how other coroutine builders (launch, async, etc) operate. It looks that this is going to be less-surprising behavior.

[elizarov]

Let me clarify the consistency here. For all other coroutine builders like launch, using them with a context:

launch(ctx) { ... }

is equivalent to using them without context, but relying on withContext to change it:

launch { withContext(ctx) { ... } }

This change essentially brings the same behavior to runBlocking function.

[gildor]

I think runBlocking + optional context is better, I don’t see why we should add one more function, Kotlin default arguments allow to avoid additional method without any drawbacks