isImmutable property
isImmutable indicates if a new item can be added to the internal map via the noSuchMethod property, or the functions inherited from the map interface.
If set to true, then only the properties that were in the original map or json string passed in can be used.
If set to false, then calling o.blah="123" will create a new blah property if it didn't already exist.
Set to true by default when a JsonObjectLite is created
with JsonObjectLite.fromJsonString() or JsonObjectLite.fromMap().
The default constructor JsonObjectLite(), sets this value to
false so properties can be added.
Implementation
bool? isImmutable;