Additional freezing tests

Trying to figure out #1745

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

@@ -0,0 +1,43 @@
+/*
+ * 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.test.*
+import kotlin.native.concurrent.*
+
+class FreezingTest : TestBase() {
+    @Test
+    fun testFreezeWithContextOther() = runTest {
+        // create a mutable object referenced by this lambda
+        val mutable = mutableListOf<Int>()
+        // run a child coroutine in another thread
+        val result = withContext(Dispatchers.Default) { "OK" }
+        assertEquals("OK", result)
+        // ensure that objects referenced by this lambda were not frozen
+        assertFalse(mutable.isFrozen)
+        mutable.add(42) // just to be 100% sure
+    }
+
+    @Test
+    fun testNoFreezeLaunchSame() = runTest {
+        // create a mutable object referenced by this lambda
+        val mutable1 = mutableListOf<Int>()
+        // this one will get captured into the other thread's lambda
+        val mutable2 = mutableListOf<Int>()
+        val job = launch { // launch into the same context --> should not freeze
+            assertEquals(mutable1.isFrozen, false)
+            assertEquals(mutable2.isFrozen, false)
+            val result = withContext(Dispatchers.Default) {
+                assertEquals(mutable2.isFrozen, true) // was frozen now
+                "OK"
+            }
+            assertEquals("OK", result)
+            assertEquals(mutable1.isFrozen, false)
+        }
+        job.join()
+        assertEquals(mutable1.isFrozen, false)
+        mutable1.add(42) // just to be 100% sure
+    }
+}