Add insertBefore test which I wrote from Hixie's fuzzer

sky/tests/dom/insertBefore-expected.txt

@@ -0,0 +1,6 @@
+unittest-suite-wait-for-done
+PASS: should throw with invalid arguments
+
+All 1 tests passed.
+unittest-suite-success
+DONE

sky/tests/dom/insertBefore.dart

@@ -0,0 +1,25 @@
+import "../resources/dom_utils.dart";
+import "../resources/third_party/unittest/unittest.dart";
+import "../resources/unit.dart";
+
+import "dart:sky";
+
+void main() {
+  initUnit();
+
+  Document document = new Document();
+
+  test("should throw with invalid arguments", () {
+    var parent = document.createElement("div");
+    var child = document.createElement("div");
+    parent.appendChild(child);
+    // TODO(eseidel): This should throw!
+    // expect(() {
+    //   parent.insertBefore([parent]);
+    // }, throws);
+    expect(() {
+      child.insertBefore([parent]);
+    }, throws);
+  });
+
+}