Add assert.

spec.html

@@ -13657,6 +13657,7 @@ <h1>Runtime Semantics: EvaluateExpressionKeyPropertyAccess( _baseValue_, _expres
       <h1>Runtime Semantics: EvaluateIdentifierKeyPropertyAccess( _baseValue_, _identifierName_, _strict_ )</h1>
       <p>The abstract operation EvaluateIdentifierKeyPropertyAccess takes as arguments a value _baseValue_, a Parse Node _identifierName_, and a Boolean argument _strict_. It performs the following steps:</p>
       <emu-alg>
+        1. Assert: _identifierName_ is an |IdentifierName|
         1. Let _bv_ be ? RequireObjectCoercible(_baseValue_).
         1. Let _propertyNameString_ be StringValue of _identifierName_.
         1. Return a value of type Reference whose base value component is _bv_, whose referenced name component is _propertyNameString_, and whose strict reference flag is _strict_.