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