anchor.diff:      enqueuer = new EnqueueTask(this);
       tasks = [scanner, dietParser, parser, resolver, checker,
                unparseValidator, constantHandler, enqueuer];
  +    tasks.addAll(backend.tasks);
     }
   
     Universe get codegenWorld() => enqueuer.codegen.universe;
  
positive.code:   SsaOptimizerTask optimizer;
    SsaCodeGeneratorTask generator;
    CodeEmitterTask emitter;
  
    JavaScriptBackend(Compiler compiler)
        : emitter = new CodeEmitterTask(compiler),
  
negative.code:   abstract String codegen(WorkItem work);
    abstract void processNativeClasses(world, libraries);
    abstract void assembleProgram();
    abstract List<CompilerTask> get tasks();
  }
  
  class JavaScriptBackend extends Backend {
  