Use NodeDispatcher on ReactNative

Fixes #236

Author: elizarov

js/kotlinx-coroutines-core-js/src/main/kotlin/kotlinx/coroutines/experimental/CoroutineContext.kt

@@ -36,14 +36,25 @@ public actual object Unconfined : CoroutineDispatcher() {
     override fun toString(): String = "Unconfined"
 }
 
+private external val navigator: dynamic
+private const val UNDEFINED = "undefined"
+
 /**
  * This is the default [CoroutineDispatcher] that is used by all standard builders like
  * [launch], [async], etc if no dispatcher nor any other [ContinuationInterceptor] is specified in their context.
  */
 @Suppress("PropertyName", "UnsafeCastFromDynamic")
 public actual val DefaultDispatcher: CoroutineDispatcher = when {
+    // Check if we are running under ReactNative. We have to use NodeDispatcher under it.
+    // The problem is that ReactNative has a `window` object with `addEventListener`, but it does not  really work.
+    // For details see https://github.com/Kotlin/kotlinx.coroutines/issues/236
+    // The check for ReactNative is based on https://github.com/facebook/react-native/commit/3c65e62183ce05893be0822da217cb803b121c61
+    jsTypeOf(navigator) != UNDEFINED && navigator != null && navigator.product == "ReactNative" ->
+        NodeDispatcher()
     // Check if we are in the browser and must use window.postMessage to avoid setTimeout throttling
-    jsTypeOf(window) != "undefined" && jsTypeOf(window.asDynamic().addEventListener) != "undefined" -> window.asCoroutineDispatcher()
+    jsTypeOf(window) != UNDEFINED && window.asDynamic() != null && jsTypeOf(window.asDynamic().addEventListener) != UNDEFINED ->
+        window.asCoroutineDispatcher()
+    // Fallback to NodeDispatcher when browser environment is not detected
     else -> NodeDispatcher()
 }