setPropertyWithSymbol method
ES6: Sets a property with a symbol key and tracks the symbol
Implementation
void setPropertyWithSymbol(String stringKey, JSValue value, JSSymbol symbol) {
final key = symbol.propertyKey;
// Track the symbol for Object.getOwnPropertySymbols()
_symbolKeys[key] = symbol;
// Set the property using the unique key
setProperty(key, value);
}