addAll method
override
Adds all key/value pairs of other to this map.
If a key of other is already in this map, its value is overwritten.
The operation is equivalent to doing this[key] = value for each key and associated value in other. It iterates over other, which must therefore not change during the iteration.
Implementation
void addAll(Map<String, dynamic> other) {
throw new UnsupportedError("Not supported");
}
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https://api.dart.dev/stable/2.13.0/dart-html/MidiOutputMap/addAll.html