The short constructor notation lets the parameter values flow directly into the properties:

BankAccount(this.owner, this.number, this.balance) { … }

The this keyword is necessary here and refers to the actual object (being constructed), but it is rarely used elsewhere (only when there is a name conflict). Initialization of instance variables can also be done in the so-called initializer list, like in this shorter version of the constructor:

BankAccount(this.owner, this.number, this.balance): dateCreated = new DateTime.now();

The variables are initialized after the colon (:) and are separated by a comma. You cannot use the this keyword in the initializer expression...