Revert "Use check instead of assert in export-unexported-dep"

This reverts commit 480eda0f. The commit
adding the assert keyword has been restored.

src/test/run-pass/export-unexported-dep.rs

@@ -15,7 +15,7 @@ fn f() -> t {
   }
 
   fn g(t v) {
-    check (v == t1);
+    assert v == t1;
   }
 }