anchor.diff:  @Task('Build the `web/index.html` entrypoint')
   build() {
     var args = context.invocation.arguments;
  -  var dart2jsArgs = {
  +  var compilerArgs = {
       if (args.hasOption(_preNullSafetyServerUrlOption))
         preNullSafetyServerUrlEnvironmentVar:
             args.getOption(_preNullSafetyServerUrlOption),
  
positive.code: String _formatDart2jsArgs(Map<String, String> args) {
    var values = args.entries.map((entry) => '"-D${entry.key}=${entry.value}"');
    return '[${values.join(',')}]';
  }
  
  @Task()
  
negative.code:   PubApp.local('build_runner').run([
      'build',
      if (!args.hasFlag(_debugFlag)) '-r',
      if (compilerArgs.isNotEmpty)
        '--define=build_web_compilers:entrypoint='
            'dart2js_args=${_formatDart2jsArgs(compilerArgs)}',
      if (compilerArgs.isNotEmpty)
        '--define=build_web_compilers:ddc='
            'environment=${_formatDdcArgs(compilerArgs)}',
      '-o',
      'web:build',
      '--delete-conflicting-outputs',
  