Check for JSDOM first.

Fixes #74

Author: armanbilge

core/src/main/scala/org/scalajs/macrotaskexecutor/MacrotaskExecutor.scala

@@ -84,7 +84,21 @@ object MacrotaskExecutor extends ExecutionContextExecutor {
         ()
       }
 
-      if (canUsePostMessage()) {
+      if (
+        js.typeOf(
+          js.Dynamic.global.navigator
+        ) != Undefined && js.Dynamic.global.navigator.userAgent
+          .asInstanceOf[js.UndefOr[String]]
+          .exists(_.contains("jsdom"))
+      ) {
+        val setImmediate =
+          js.Dynamic.global.Node.constructor("return setImmediate")()
+
+        { k =>
+          setImmediate(k)
+          ()
+        }
+      } else if (canUsePostMessage()) {
         // postMessage is what we use for most modern browsers (when not in a webworker)
 
         // generate a unique messagePrefix for everything we do
@@ -134,20 +148,6 @@ object MacrotaskExecutor extends ExecutionContextExecutor {
           channel.port2.postMessage(handle)
           ()
         }
-      } else if (
-        js.typeOf(
-          js.Dynamic.global.navigator
-        ) != Undefined && js.Dynamic.global.navigator.userAgent
-          .asInstanceOf[js.UndefOr[String]]
-          .exists(_.contains("jsdom"))
-      ) {
-        val setImmediate =
-          js.Dynamic.global.Node.constructor("return setImmediate")()
-
-        { k =>
-          setImmediate(k)
-          ()
-        }
       } else {
         // we don't try to look for process.nextTick since scalajs doesn't support old node
         // we're also not going to bother fast-pathing for IE6; just fall through