[es6] Perform the IsConstructor test in GetSuperConstructor.

test/webkit/class-syntax-extends-expected.txt

Index: test/webkit/class-syntax-extends-expected.txt
diff --git a/test/webkit/class-syntax-extends-expected.txt b/test/webkit/class-syntax-extends-expected.txt
index 5d0dca7263688ba636f14d738dc6e2b5b61d2329..2f77544453c9ee640239241e1778b2f15ebf37da 100644
--- a/test/webkit/class-syntax-extends-expected.txt
+++ b/test/webkit/class-syntax-extends-expected.txt
@@ -60,7 +60,7 @@ PASS x = {}; new (class extends undefined { constructor () { return x; } }) thre
 PASS y = 12; new (class extends undefined { constructor () { return y; } }) threw exception TypeError: Class extends value undefined is not a constructor or null.
 PASS class x {}; new (class extends null { constructor () { return new x; } }) instanceof x is true
 PASS new (class extends null { constructor () { this; } }) threw exception ReferenceError: this is not defined.
-PASS new (class extends null { constructor () { super(); } }) threw exception TypeError: super is not a constructor.
+PASS new (class extends null { constructor () { super(); } }) threw exception TypeError: Super constructor null of anonymous class is not a constructor.
 PASS x = {}; new (class extends null { constructor () { return x } }) is x
 PASS y = 12; new (class extends null { constructor () { return y; } }) threw exception TypeError: Derived constructors may only return object or undefined.
 PASS class x {}; new (class extends null { constructor () { return new x; } }) instanceof x is true