Easy way to inject js in tinymce iframe?

WordPress injects its own external script files in the TinyMCE editor iframe, see for example the wp-includes/js/tinymce/plugins/wpembed/plugin.js file:

(function ( tinymce ) {
    'use strict';

    tinymce.PluginManager.add( 'wpembed', function ( editor, url ) {
        editor.on( 'init', function () {
            var scriptId = editor.dom.uniqueId();

            var scriptElm = editor.dom.create( 'script', {
                id: scriptId,
                type: 'text/javascript',
                src: url + '/../../../wp-embed.js'
            } );

            editor.getDoc().getElementsByTagName( 'head' )[ 0 ].appendChild( scriptElm );
        } );
    } );
})( window.tinymce );

The plugin.js file is loaded in TinyMCE using the tiny_mce_plugins filter.