Look up the value of key, or add a new value if it isn't there.
Returns the value associated to key, if there is one.
Otherwise calls ifAbsent to get a new value, associates key to
that value, and then returns the new value.
Map<String, int> scores = {'Bob': 36};
for (var key in ['Bob', 'Rohan', 'Sophena']) {
scores.putIfAbsent(key, () => key.length);
}
scores['Bob']; // 36
scores['Rohan']; // 5
scores['Sophena']; // 7
Calling ifAbsent must not add or remove keys from the map.
dynamic putIfAbsent(String key, dynamic ifAbsent()) {
throw new UnsupportedError("Not supported");
}