Add context to generated named and indexed property operations.

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

Index: LayoutTests/fast/dom/script-tests/dataset-xhtml.js
diff --git a/LayoutTests/fast/dom/script-tests/dataset-xhtml.js b/LayoutTests/fast/dom/script-tests/dataset-xhtml.js
index fd7c3187177344ce74c6e7fbf013b4c637eea105..839b74d6aba9372c54e4466ff07d7fec495e41d3 100644
--- a/LayoutTests/fast/dom/script-tests/dataset-xhtml.js
+++ b/LayoutTests/fast/dom/script-tests/dataset-xhtml.js
@@ -48,8 +48,8 @@ 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)