Fix hit-testing of elements with the same z-index.

sky/tests/layout/document-elementFromPoint-absolute-position.sky

+<style>
+postive-z-above,
+postive-z-below,
+zero-z-above,
+zero-z-below,
+no-z-above,
+no-z-below,
+postive-z-after {
+  position: absolute;
+  display: block;
+  left: 0;
+  right: 0;
+  top: 0;
+  bottom: 0;
+  background-color: blue;
+}
+
+no-z-below {
+  top: 50px;
+  background-color: green;
+}
+zero-z-above {
+  z-index: 0;
+  top: 100px;
+  background-color: red;
+}
+zero-z-below {
+  z-index: 0;
+  top: 150px;
+  background-color: salmon;
+}
+postive-z-above {
+  z-index: 1;
+  top: 200px;
+  background-color: yellow;
+}
+postive-z-below {
+  z-index: 1;
+  top: 250px;
+  background-color: pink;
+}
+postive-z-after {
+  z-index: 1;
+  top: 300px;
+  background-color: orange;
+}
+</style>
+<postive-z-above layer yellow></postive-z-above>
+<postive-z-below layer pink></postive-z-below>
+<no-z-above no-layer blue></no-z-above>
+<no-z-below no-layer green></no-z-below>
+<zero-z-above layer red></zero-z-above>
+<zero-z-below layer salmon></zero-z-below>
+<postive-z-after layer orange></postive-z-after>
+<script>
+import "../resources/third_party/unittest/unittest.dart";
+import "../resources/unit.dart";
+
+import "dart:sky";
+
+void main() {
+  initUnit();
+
+  test("should hit test top item", () {
+    expect(document.elementFromPoint(100, 25).tagName, equals('no-z-above'));
+  });
+
+  test("should hit test second", () {
+    expect(document.elementFromPoint(100, 75).tagName, equals('no-z-below'));
+  });
+
+  test("should hit test third", () {
+    expect(document.elementFromPoint(100, 125).tagName, equals('zero-z-above'));
+  });
+
+  test("should hit test fourth", () {
+    expect(document.elementFromPoint(100, 175).tagName, equals('zero-z-below'));
+  });
+
+  test("should hit test fifth", () {
+    expect(document.elementFromPoint(100, 225).tagName, equals('postive-z-above'));
+  });
+
+  test("should hit test sixth", () {
+    expect(document.elementFromPoint(100, 275).tagName, equals('postive-z-below'));
+  });
+
+  test("should hit test seventh", () {
+    expect(document.elementFromPoint(100, 325).tagName, equals('postive-z-after'));
+  });
+}
+</script>