Fixed tether requirement for UMD.

[neochief]

It does not seem possible to use tooltips withing UMD environment because of hard windows.Tether check. I've added proper import of tether, which should eliminate the problem.

[cvrebert]

It's planned to resolve this properly as part of fixing #19017.

[neochief]

Thanks, I haven't found that one while I was searching for similar issues.

[mister-good-deal]

Hi, got the error using requirejs and a shim declaration for tether requirement.

require.config(
    {
        'paths' : {
            'require'  : 'vendors/require',
            'bootstrap': 'vendors/bootstrap',
            'tether'   : 'vendors/tether',
            'jquery'   : 'vendors/jquery'
        },
        'shim'  : {
            'bootstrap': {
                'deps': ['jquery', 'tether']
            }
        }
    }
);

Is this fix merged in dev branch ? I'm using bower to required bootstrap 4 with

{
    "dependencies": {
        "jquery"         : "^2.2.4",
        "bootstrap"      : "https://github.com/twbs/bootstrap.git#v4-dev",
        "font-awesome"   : "^4.6.3",
        "requirejs"      : "^2.2.0",
        "tether"         : "^1.3.3"
    }
}