Do proper checkcast in takeEventLoop, throw ISE instead of TCE

Fixes #920

Author: qwwdfsad

native/kotlinx-coroutines-core-native/src/CoroutineContext.kt

@@ -8,7 +8,7 @@ import kotlin.coroutines.*
 import kotlinx.coroutines.internal.*
 
 private fun takeEventLoop(): EventLoopImpl =
-    ThreadLocalEventLoop.currentOrNull() as EventLoopImpl ?:
+    ThreadLocalEventLoop.currentOrNull() as? EventLoopImpl ?:
         error("There is no event loop. Use runBlocking { ... } to start one.")
 
 internal object DefaultExecutor : CoroutineDispatcher(), Delay {

native/kotlinx-coroutines-core-native/test/DelayExceptionTest.kt

@@ -0,0 +1,28 @@
+/*
+ * Copyright 2016-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.coroutines
+
+import kotlin.coroutines.*
+import kotlin.test.Test
+import kotlin.test.assertTrue
+
+class DelayExceptionTest {
+    private object Dispatcher : CoroutineDispatcher() {
+        override fun isDispatchNeeded(context: CoroutineContext): Boolean = true
+        override fun dispatch(context: CoroutineContext, block: Runnable) { block.run() }
+    }
+
+    private lateinit var exception: Throwable
+
+
+    @Test
+    fun testThrowsTce() {
+        CoroutineScope(Dispatcher + CoroutineExceptionHandler { _, e -> exception = e }).launch {
+            delay(10)
+        }
+
+        assertTrue(exception is IllegalStateException)
+    }
+}