anchor.diff:  List<jsAst.Statement> buildTearOffCode(
       CompilerOptions options, Emitter emitter, CommonElements commonElements) {
     FunctionEntity closureFromTearOff = commonElements.closureFromTearOff;
  -  jsAst.Expression closureFromTearOffAccessExpression;
  -  if (closureFromTearOff != null) {
  -    closureFromTearOffAccessExpression =
  -        emitter.staticFunctionAccess(closureFromTearOff);
  -  } else {
  -    // Default values for mocked-up test libraries.
  -    closureFromTearOffAccessExpression =
  -        js(r'''function() { throw "Helper 'closureFromTearOff' missing." }''');
  -  }
  +
  +  jsAst.Expression closureFromTearOffAccessExpression =
  +      emitter.staticFunctionAccess(closureFromTearOff);
   
     jsAst.Statement instanceTearOffGetter;
     if (options.features.useContentSecurityPolicy.isEnabled) {
  
positive.code: 
      jsAst.Expression code;
      if (field.isElided) {
        ConstantValue constantValue = field.constantValue;
        assert(
            constantValue != null, "No constant value for elided field: $field");
        if (constantValue == null) {
          // This should never occur because codegen member usage is now limited
          // by closed world member usage. In the case we've missed a spot we
          // cautiously generate a null constant.
          constantValue = NullConstantValue();
        }
        code = js("function() { return #; }",
            _emitter.constantReference(constantValue));
      } else {
        String template;
        if (field.needsInterceptedGetterOnReceiver) {
  
negative.code: 
    ClassStubGenerator(this._emitter, this._commonElements, this._namer,
        this._codegenWorld, this._closedWorld,
        {required this.enableMinification});
  
    InterceptorData get _interceptorData => _closedWorld.interceptorData;
  
  