dart - What is the best way to set the default instance for a final private field with optional parameter? -

given class final private field _bar:

class foo() {  final bar _bar;      foo({bar bar}) : _bar = bar {          //error: list cannot used setter, final        _bar = new bar();     }  } 

attempting set in parameter list results in error

//error: default values of object must constant foo({bar bar: new bar()}) : _bar = bar .. 

i'd keep optional parameter can inject mocks during unit tests. best approach this?

class foo {   final bar _bar;   foo({bar bar}) : _bar = bar != null ? bar : new bar(); }