Implement platform deleters per spec.

LayoutTests/fast/dom/script-tests/dataset.js

 description("This tests element.dataset.");
 
 function testGet(attr, expected)
 {
     var d = document.createElement("div");
     d.setAttribute(attr, "value");
     return d.dataset[expected] == "value";
 }
 
 shouldBeTrue("testGet('data-foo', 'foo')");
@@ skipping 98 matching lines @@
 shouldBeTrue("testDelete('data-', '')");
 shouldBeTrue("testDelete('data-\xE0', '\xE0')");
 shouldBeTrue("testDelete('data-33', '33')");
 shouldBeTrue("testDelete('data-00033', '00033')");
 shouldBeTrue("testDelete('data-r2', 'r2')");
 shouldBeTrue("testDelete('data-2r', '2r')");
 shouldBeTrue("testDelete('data-r-2', 'r-2')");
 shouldBeTrue("testDelete('data--r-2-', 'R-2-')");
 shouldBeTrue("testDelete('data--r-2r', 'R-2r')");
 shouldBeTrue("testDelete('data--r-2-----r', 'R-2----R')");
+
+// The element.dataset deleter is only applied to properties
+// that are present; check that any underlying native property
+// is deleted instead.
+function testNativeDelete(prop, isConfigurable)
+{
+    var d = document.createElement("div");
+    Object.defineProperty(d.dataset, prop, {configurable: isConfigurable, value: "native_value"});
+    delete d.dataset[prop];
+    return isConfigurable ? !(prop in d.dataset) : (d.dataset[prop] === "native_value");
+}
+
+shouldBeTrue("testNativeDelete('-r-2-', false)");
+shouldBeTrue("testNativeDelete('foo', true)");
+
 debug("");
 
 shouldBeFalse("testDelete('dummy', '-foo')");
 debug("");
 
 function testForIn(array)
 {
     var d = document.createElement("div");
     for (var i = 0; i < array.length; ++i) {
         d.setAttribute(array[i], "value");
@@ skipping 66 matching lines @@
 shouldBe("div.getAttribute('data-a-500k')", "'updated'");
 
 debug("");
 debug("Set null:");
 
 var d = document.createElement("div");
 d.dataset.foo = null;
 shouldBe("d.dataset.foo", "'null'");
 
 debug("");