Update mergeMaps making it more flexible

[bambinoua]

The updated version of mergeMaps function allows to merge maps with removal functionality.

final map1 = {'a': 'a'};
final map2 = {'b': 'b'};
final map3 = {'c': 'c'};
final map12 = mergeMaps(map1, map2);                     // {'a': 'a', 'b': 'b'}
final map123 = mergeMaps(map12, map3);                   // {'a': 'a', 'b': 'b', 'c': 'c'}
final map123_reduced1 = mergeMaps(map123, {'c': null});  // {'a': 'a', 'b': 'b'}
final map4 = {'a': 'a', 'b': 'b'};
final map123_reduced2 = mergeMaps(map123, map4, 
 removeDifference: true);                                // {'a': 'a', 'b': 'b'}

[natebosch]

This new behavior seems pretty specialized. I don't think we should have behavior dependent on treating null value in some special way - it's not typical to treat present with a null value differently from an absent key, but we give it a special meaning here it will be breaking for anyone that is doing it already.

The removeDifference could also be handled outside this method mergeMaps(aMap, bMap)..removeWhere((k, v) => !bMap.containsKey(k)). With that behavior it's also feasible to write it with a map literal { for (var MapEntry(:key, :value) in bMap.entries) key: aMap.containsKey(key) ? _merge(aMap[key], value) : value}

I don't think it's worth making mergeMaps more complicated for this. cc @lrhn in case you have different preferences.