Add context to generated named and indexed property operations.

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

Index: LayoutTests/fast/dom/script-tests/dataset.js
diff --git a/LayoutTests/fast/dom/script-tests/dataset.js b/LayoutTests/fast/dom/script-tests/dataset.js
index bf7b305b85458fb5b66c38deda4516ff62469ea1..6b8281fb4984044172dbdbd10dae86dfba05e131 100644
--- a/LayoutTests/fast/dom/script-tests/dataset.js
+++ b/LayoutTests/fast/dom/script-tests/dataset.js
@@ -89,8 +89,8 @@ 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)