Bug Report

Current Behavior
When polyfilling down to IE9, @Babel/polyfill seems to lack an implementation for the MutationObserver object (see screenshot).

Input Code
– private, proprietary –

Expected behavior/code
MutationObserver should be polyfilled.

Babel Configuration (.babelrc, package.json, cli command)

webpack.config.js

{
  test: /\.js$/i,
  include: [
    /[/\\]node_modules[/\\]lit-html[/\\]/i,
    /[/\\]node_modules[/\\]@babel[/\\]/i,
    /[/\\]node_modules[/\\]@webcomponents[/\\]/i
  ],
  use: {
    loader: 'babel-loader',
    options: {
      presets: ['@babel/preset-env']
    }
  }
}

.babelrc

{
	useBuiltIns: false
}

browserslist

ie 9

main.ts

import "@babel/polyfill"

import "@webcomponents/webcomponentsjs/webcomponents-bundle";
import '@webcomponents/webcomponentsjs/custom-elements-es5-adapter.js';

Environment

• Babel version(s):
  "@babel/core": "^7.4.3"
  "@babel/polyfill": "^7.4.3"
  "@babel/preset-env": "^7.4.3"
• Node/npm version: v10.15.3/6.4.1
• OS: Windows 10x64
• How you are using Babel: webpack