Normative: Reject SharedArrayBuffer receiver in ArrayBuffer.prototype.immutable getter (#51)

Fixes #49

Author: gibson042

spec.emu

@@ -830,6 +830,7 @@ contributors: Mark S. Miller, Richard Gibson
           <emu-alg>
             1. Let _O_ be the *this* value.
             1. Perform ? RequireInternalSlot(_O_, [[ArrayBufferData]]).
+            1. If IsSharedArrayBuffer(_O_) is *true*, throw a *TypeError* exception.
             1. Return IsImmutableBuffer(_O_).
           </emu-alg>
         </emu-clause>