code: function createSequencialCommandExecutor (commands, workingDir, envVars, watchMode, host) {
      const source = Observable.from([{
          cwd: workingDir,
          env: envVars,
          exitCode: 0
      }]);
  
      return commands.reduce((prev, command) =>
          createCommandExecutor(command, prev, watchMode, host), source);
  }
docs: Create an Observable that would execute a sequence of commands.
  
  @param commands {string[]} A sequence of commands to execute
  @param workingDir {string} Initial Working Directory
  @param envVars {Object} Initial environment variables
  @param watchMode {boolean} Whether to execute watchers or not
  @param host {{spawn: Function, log: Function}} Mockable host environment (mimicking child_process.spawn() and console.log())
  @returns Observable