Add context to generated named and indexed property operations.

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 71 matching lines @@
     var d = document.createElement("div");
     d.dataset[prop] = "value";
     return d.getAttribute(attr) === null;
 }
 
 shouldBeTrue("testIsNull('0123', 'data-123')");
 shouldBeTrue("testIsNull('123', 'data-0123')");
 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 106 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("");