Added a test case for #506

Author: Eric Holk

src/test/run-pass/issue-506.rs

@@ -0,0 +1,12 @@
+/*
+  A reduced test case for Issue #506, provided by Rob Arnold.
+*/
+
+native "rust" mod rustrt {
+  fn task_yield();
+}
+
+fn main() {
+  spawn rustrt::task_yield();
+}
+