Add a compile-fail test for attempts to extend non-objects.

src/test/compile-fail/extend-non-object.rs

@@ -0,0 +1,13 @@
+//error-pattern:x does not have object type
+use std;
+
+fn main() {
+    auto x = 3;
+
+    auto anon_obj = obj {
+        fn foo() -> int {
+            ret 3;
+        }
+        with x
+    };
+}