Optimize startCoroutine impls, move them to common stdlib

Do not wrap initial continuation for startCoroutine in SafeContinuation

This changes leaves only internal declarations and intrinsics as platform
dependent parts of the coroutine library, the rest parts (public API)
is implemented through them in common module

js/js.libraries/src/core/coroutines.kt

@@ -18,38 +18,14 @@ package kotlin.coroutines.experimental
 
 import kotlin.coroutines.experimental.intrinsics.*
 
-/**
- * Creates coroutine with receiver type [R] and result type [T].
- * This function creates a new, fresh instance of suspendable computation every time it is invoked.
- * To start executing the created coroutine, invoke `resume(Unit)` on the returned [Continuation] instance.
- * The [completion] continuation is invoked when coroutine completes with result of exception.
- */
-@SinceKotlin("1.1")
-public fun <R, T> (suspend R.() -> T).createCoroutine(
+internal fun <R, T> (suspend R.() -> T).createCoroutineInternal(
         receiver: R,
         completion: Continuation<T>
-): Continuation<Unit> =
-        SafeContinuation(
-                this.asDynamic()(receiver, completion, true),
-                COROUTINE_SUSPENDED
-        )
+): Continuation<Unit> = this.asDynamic()(receiver, completion, true)
 
-/**
- * Creates coroutine without receiver and with result type [T].
- * This function creates a new, fresh instance of suspendable computation every time it is invoked.
- * To start executing the created coroutine, invoke `resume(Unit)` on the returned [Continuation] instance.
- * The [completion] continuation is invoked when coroutine completes with result of exception.
- */
-@SinceKotlin("1.1")
-public fun <T> (suspend () -> T).createCoroutine(
+internal fun <T> (suspend () -> T).createCoroutineInternal(
         completion: Continuation<T>
-): Continuation<Unit> =
-        SafeContinuation(
-                this.asDynamic()(completion, true),
-                COROUTINE_SUSPENDED
-        )
-
-// ------- internal stuff -------
+): Continuation<Unit> = this.asDynamic()(completion, true)
 
 @JsName("CoroutineImpl")
 internal abstract class CoroutineImpl(private val resultContinuation: Continuation<Any?>) : Continuation<Any?> {