anchor.diff:  
     TypeMaskSystem get typeSystem => lattice.typeSystem;
   
  +  JavaScriptBackend get backend => typeSystem.backend;
  +
     AbstractValue get nothing => lattice.nothing;
   
     AbstractValue nonConstant([TypeMask type]) => lattice.nonConstant(type);
  
positive.code:   void visit(Node node) { node.accept(this); }
  
    void visitFunctionDefinition(FunctionDefinition node) {
      if (node.thisParameter != null) {
        setValue(node.thisParameter,
                 nonConstant(typeSystem.getReceiverType(node.element)));
      }
      node.parameters.forEach(visit);
      push(node.body);
    }
  
  
negative.code:   }
  
    void visitMutableVariable(MutableVariable node) {
    }
  
    void visitParameter(Parameter node) {
    }
  
    void visitContinuation(Continuation node) {
  