From the ES6 specification:
> ## PropertyDefinition : IdentifierReference
>
> 1. Let propName be StringValue of IdentifierReference.
> 2. Let exprValue be the result of evaluating IdentifierReference.
> 3. ReturnIfAbrupt(exprValue).
> 4. Let propValue be GetValue(exprValue).
> 5. ReturnIfAbrupt(propValue).
> 6. Assert: enumerable is true.
> 7. Return CreateDataPropertyOrThrow(object, propName, propValue).
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