anchor.diff:    visitChildren(Visitor v) {}
     accept(ConstantVisitor v) => v.visitNullConstant(this);
     acceptReference(Visitor v) => v.visitNullConstantReference(this);
  +
  +  DartType getType(TypeEnvironment types) => types.nullType;
   }
   
   class BoolConstant extends PrimitiveConstant<bool> {
  
positive.code:   visitChildren(Visitor v) {}
    accept(ConstantVisitor v) => v.visitBoolConstant(this);
    acceptReference(Visitor v) => v.visitBoolConstantReference(this);
  }
  
  class IntConstant extends PrimitiveConstant<int> {
  
negative.code:   bool operator ==(Object other) {
      return other is TypeLiteralConstant && other.type == type;
    }
  
    DartType getType(TypeEnvironment types) => types.typeType;
  }
  
  // ------------------------------------------------------------------------
  