operator []= method
void operator []=(Sets the value of this Expando's property on the given object. Properties can effectively be removed again by setting their value to null.
The object must not be a number, a string, a boolean or null.
Source
external void operator []=(Object object, T value);
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https://api.dartlang.org/stable/1.24.3/dart-core/Expando/operator_put.html