Add test for subclassing bound functions (#720)

The semantics under test have been incorrectly implemented by the
SpiderMonkey engine.

test/language/statements/class/subclass/bound-function.js

+// Copyright (C) 2016 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-runtime-semantics-classdefinitionevaluation
+es6id: 14.5.14
+description: SuperClass may be a bound function object
+info: |
+  [...]
+  5. If ClassHeritageopt is not present, then
+     [...]
+  6. Else,
+     [...]
+     e. If superclass is null, then
+        [...]
+     f. Else if IsConstructor(superclass) is false, throw a TypeError exception.
+     g. Else,
+        i. Let protoParent be ? Get(superclass, "prototype").
+        ii. If Type(protoParent) is neither Object nor Null, throw a TypeError
+            exception.
+        iii. Let constructorParent be superclass.
+     [...]
+---*/
+
+var bound = function() {}.bind();
+bound.prototype = {};
+
+class C extends bound {}
+
+assert.sameValue(Object.getPrototypeOf(new C()), C.prototype);