Generate a bit less code to handle failed arity checks.

LayoutTests/fast/dom/MutationObserver/mutation-observer-constructor-expected.txt

Index: LayoutTests/fast/dom/MutationObserver/mutation-observer-constructor-expected.txt
diff --git a/LayoutTests/fast/dom/MutationObserver/mutation-observer-constructor-expected.txt b/LayoutTests/fast/dom/MutationObserver/mutation-observer-constructor-expected.txt
index f13ef82e7ed770797a1e377ad02b280443b0c8e9..a444a97918c767b0a1719236722e2ed09bd4f670 100644
--- a/LayoutTests/fast/dom/MutationObserver/mutation-observer-constructor-expected.txt
+++ b/LayoutTests/fast/dom/MutationObserver/mutation-observer-constructor-expected.txt
@@ -8,10 +8,10 @@ PASS typeof WebKitMutationObserver.prototype.observe is "function"
 PASS typeof WebKitMutationObserver.prototype.disconnect is "function"
 PASS typeof observer.observe is "function"
 PASS typeof observer.disconnect is "function"
-PASS new MutationObserver({ handleEvent: function() {} }) threw exception TypeError: Callback argument must be a function.
-PASS new MutationObserver({}) threw exception TypeError: Callback argument must be a function.
-PASS new MutationObserver(42) threw exception TypeError: Callback argument must be a function.
-PASS new MutationObserver("foo") threw exception TypeError: Callback argument must be a function.
+PASS new MutationObserver({ handleEvent: function() {} }) threw exception TypeError: Failed to construct 'MutationObserver': Callback argument must be a function.
+PASS new MutationObserver({}) threw exception TypeError: Failed to construct 'MutationObserver': Callback argument must be a function.
+PASS new MutationObserver(42) threw exception TypeError: Failed to construct 'MutationObserver': Callback argument must be a function.
+PASS new MutationObserver("foo") threw exception TypeError: Failed to construct 'MutationObserver': Callback argument must be a function.
 PASS successfullyParsed is true
 
 TEST COMPLETE