Babel 7

Created by: hzoo

👋 Bootstrap team! Making a PR to change to v7 if desired! (instead of by accident via https://github.com/twbs/bootstrap/pull/22727, hopefully we can fix that kind of dependency issue soon)

• Update packages to Babel 7
• Use https://github.com/babel/babel-preset-env instead of preset-es2015 (just deprecated it), has the same behavior if you don't specify a target
• feel free to edit the pr

Diff

• New loose option for classes removes a bunch of helper checks, so they won't be in the bundle/files anymore, so a smaller output. Via https://github.com/babel/babel/pull/4850

• rm'classCallCheck', 'inherits', 'possibleConstructorReturn'

• add inheritsLoose

• Aliases the class prototype via https://github.com/babel/babel/pull/5560

• babel-generator defaults to " double quotes now via https://github.com/babel/babel/pull/5824

• there are some newline changes because Babel won't read tokens from the AST anymore, so it may look different but it should compile faster (faster parse/print) via https://github.com/babel/babel/pull/5833
• Comments from dead code elim for a minifier if using exports. via https://github.com/babel/babel/pull/6209

• default for template literals is using concat instead of +, can use loose to go back to + via https://github.com/babel/babel/pull/6098