Add context to generated named and indexed property operations.

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

 description("This tests element.dataset for XHTML.");
 
 function testGet(attr, expected)
 {
     var d = document.createElement("div");
     d.setAttribute(attr, "value");
     return d.dataset[expected] == "value";
 }
 
 shouldBeTrue("testGet('data-foo', 'foo')");
@@ skipping 30 matching lines @@
 shouldBeTrue("testSet('foo', 'data-foo')");
 shouldBeTrue("testSet('fooBar', 'data-foo-bar')");
 shouldBeTrue("testSet('-', 'data--')");
 shouldBeTrue("testSet('Foo', 'data--foo')");
 shouldBeTrue("testSet('-Foo', 'data---foo')");
 shouldBeTrue("testSet('', 'data-')");
 shouldBeTrue("testSet('\xE0', 'data-\xE0')");
 debug("");
 
 shouldThrow("testSet('-foo', 'dummy')", '"SyntaxError: Failed to set the \'-foo\' property on \'DOMStringMap\': \'-foo\' is not a valid property name."');
-shouldThrow("testSet('foo\x20', 'dummy')", '"InvalidCharacterError: \'data-foo\x20\' is not a valid attribute name."');
-shouldThrow("testSet('foo\uF900', 'dummy')", '"InvalidCharacterError: \'data-foo\uF900\' is not a valid attribute name."');
+shouldThrow("testSet('foo\x20', 'dummy')", '"InvalidCharacterError: Failed to set the \'foo\x20\' property on \'DOMStringMap\': \'data-foo\x20\' is not a valid attribute name."');
+shouldThrow("testSet('foo\uF900', 'dummy')", '"InvalidCharacterError: Failed to set the \'foo\uF900\' property on \'DOMStringMap\': \'data-foo\uF900\' is not a valid attribute name."');
 debug("");
 
 function testDelete(attr, prop)
 {
     var d = document.createElement("div");
     d.setAttribute(attr, "value");
     delete d.dataset[prop];
     return d.getAttribute(attr) != "value";
 }
 
@@ skipping 20 matching lines @@
     for (var item in d.dataset)
         count++;
 
     return count;
 }
 
 shouldBe("testForIn(['data-foo', 'data-bar', 'data-baz'])", "3");
 shouldBe("testForIn(['data-foo', 'data-bar', 'dataFoo'])", "2");
 shouldBe("testForIn(['data-foo', 'data-bar', 'style'])", "2");
 shouldBe("testForIn(['data-foo', 'data-bar', 'data-'])", "3");