anchor.diff:  /// continuation body.
   /// During one-pass construction a LetCont with an empty continuation body is
   /// used to represent the one-level context 'let cont k(v) = [] in E'.
  -class LetCont extends Expression implements InteriorNode {
  +class LetCont extends Expression {
     final Continuation continuation;
  -  Expression body;
  +  final Expression body;
   
     LetCont(this.continuation, this.body);
   
  
positive.code: /// Continuations are normally bound by 'let cont'.  A continuation with no
  /// parameter (or body) is used to represent a function's return continuation.
  /// The return continuation is bound by the Function, not by 'let cont'.
  class Continuation extends Definition implements InteriorNode {
    final List<Parameter> parameters;
    Expression body = null;
  
  
negative.code: abstract class Invoke {
    Selector get selector;
    List<Reference> get arguments;
  }
  
  /// Invoke a static function or static field getter/setter.
  