anchor.diff:  /// Expression of form `super.field = value`.
   ///
   /// This may invoke a setter or assign a field.
  +///
  +/// Evaluates to the value of [value].
   class SuperPropertySet extends Expression {
     Name name;
     Expression value;
  
positive.code: }
  
  /// Assign a static field or call a static setter.
  class StaticSet extends Expression {
    /// A mutable static field or a static setter.
    Member target;
  
negative.code: }
  
  /// Directly assign a field, or call a setter.
  ///
  /// Evaluates to the value of [value].
  class DirectPropertySet extends Expression {
    Expression receiver;
    Member target;
  