Re-imagined CoroutineContext as a decorator around a standard map.

FakeCoroutineContext.kt

 class CoroutineContext {
   internal val map = mutableMapOf<Key<*>, Element>()

   operator fun <E : Element> get(key: Key<E>): E? = map[key] as? E

   operator fun plus(context: CoroutineContext): CoroutineContext {
     val new = CoroutineContext()
     new.map.putAll(map)
     new.map.putAll(context.map)
     return new
   }

   interface Key<E : Element>
   interface Element
 }