Fix broadcast builder with different thread

Fixes #1831

Author: elizarov

kotlinx-coroutines-core/common/src/channels/Broadcast.kt

@@ -170,7 +170,7 @@ private class LazyBroadcastCoroutine<E>(
     channel: BroadcastChannel<E>,
     block: suspend ProducerScope<E>.() -> Unit
 ) : BroadcastCoroutine<E>(parentContext, channel, active = false) {
-    private val continuation = block.createCoroutineUnintercepted(this, this)
+    private val saved = saveLazyCoroutine(this, this, block)
 
     override fun openSubscription(): ReceiveChannel<E> {
         // open subscription _first_
@@ -181,6 +181,6 @@ private class LazyBroadcastCoroutine<E>(
     }
 
     override fun onStart() {
-        continuation.startCoroutineCancellable(this)
+        startLazyCoroutine(saved, this, this)
     }
 }

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2016-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ * Copyright 2016-2020 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
  */
 
 package kotlinx.coroutines
@@ -296,5 +296,22 @@ class WorkerDispatcherTest : TestBase() {
         finish(2)
     }
 
+    @Test
+    fun testBroadcastAsFlow() = runTest {
+        expect(1)
+        withContext(dispatcher) {
+            expect(2)
+            broadcast {
+                expect(3)
+                send("OK")
+            }.asFlow().collect {
+                expect(4)
+                assertEquals("OK", it)
+            }
+            expect(5)
+        }
+        finish(6)
+    }
+
     private data class Data(val s: String)
 }