Browserify bundle returns Uncaught ReferenceError: angular is not defined

I am using make a bundle of node_modules with gulp using browserify. This is the task:

var gulp = require('gulp'),
  g = require('gulp-load-plugins')({lazy: false});
var browserify   = require('browserify');
var packageJson = require('./package.json');
var dependencies = Object.keys(packageJson && packageJson.dependencies || {});
var source = require('vinyl-source-stream');

gulp.task('libs', function () {
  return browserify()
    .require(dependencies)
    .bundle()
    .pipe(source('libs.js'))
    .pipe(gulp.dest('./build/'));
});

This generates a bundle named libs.js and I add it to the index.html

But when I try to use angular javascript says:

Uncaught ReferenceError: angular is not defined(…)

I can see the angular.js code inside libs.js, why is not being loaded?

This is how libs.js starts:

require=(function e(t,n,r){function s(o,u){if(!n[o]){if(!t[o]){var a=typeof require=="function"&&require;if(!u&&a)return a(o,!0);if(i)return i(o,!0);var f=new Error("Cannot find module '"+o+"'");throw f.code="MODULE_NOT_FOUND",f}var l=n[o]={exports:{}};t[o][0].call(l.exports,function(e){var n=t[o][1][e];return s(n?n:e)},l,l.exports,e,t,n,r)}return n[o].exports}var i=typeof require=="function"&&require;for(var o=0;o<r.length;o++)s(r[o]);return s})({1:[function(require,module,exports){
/**
 * @license AngularJS v1.6.4
 * (c) 2010-2017 Google, Inc. http://angularjs.org
 * License: MIT
 */
(function(window, angular) {'use strict';

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