Normative: Early error for class decorators in sloppy mode (#207)

Closes #204

spec.html

@@ -147,6 +147,25 @@ <h1>Static Semantics: Early Errors</h1>
         It is a Syntax Error if |DecoratorList| is present and PropName of |MethodDefinition| is `"constructor"`.
       </li>
     </ul>
+
+
+    <emu-grammar>
+      ClassDeclaration[Yield, Await, Default] :
+        <ins>DecoratorList[?Yield, ?Await]?</ins> `class` BindingIdentifier[?Yield, ?Await] ClassTail[?Yield, ?Await]
+
+      ClassExpression[Yield, Await] :
+        <ins>DecoratorList[?Yield, ?Await]?</ins> `class` BindingIdentifier[?Yield, ?Await]? ClassTail[?Yield, ?Await]
+    </emu-grammar>
+    <ul>
+      <li>
+        It is a Syntax Error if |DecoratorList| is present and |ClassExpression| or |ClassDeclaration| is directly contained in non-strict code.
+      </li>
+    </ul>
+
+    <emu-note>
+      |DecoratorList| is always interpreted as strict mode code, as it is part of a |ClassDeclaration| or |ClassExpression|.
+    </emu-note>
+
   </emu-clause>
 </emu-clause>