* Extract for-of iterator handling to a helper
Dis greatly recudes the code size when for-of is used multiple times across the
codebase. Also, makes it easier to read what's happening in a compiled loop.
* Unify spec and loose code
* Legacy implementation fallback
* Update tmp var name
* Updates from review and after rebase
* Node modules compatibility: add ".js" extension to imported polyfills
* Update fixtures
* add extensions on calling `addDefaultImport()`, not in the function.
### `@babel/runtime`
- Added `@babel/runtime-corejs3` package and `corejs: 3` options to `@babel/plugin-transform-runtime`.
- Added support of instance methods, fixes#8928.
- Added flag `proposals` (in `corejs: { version: 3, proposals: true }` format) for support all proposals polyfills from `core-js`.
- Used separate directories in runtime for `core-js` entry points with proposals and without.
- Used `get-iterator-method` helper for getting iterators, fixes#2500.
- As a cheap bonus, added support of IE8- (except some cases of `regenerator`).
### `@babel/polyfill`
- Should be deprecated in favor of separate usage required features from `core-js` and `regenerator-runtime` with an informative message.
### `@babel/preset-env`
- Uses for built-ins data from [`core-js-compat`](https://github.com/zloirock/core-js/tree/master/packages/core-js-compat) instead of `compat-table` since information from `compat-table` [is not enough](https://github.com/zloirock/core-js/tree/master/packages/core-js-compat).
- `useBuilIns` now requires direct setting of `corejs` version option, without it will be used `2` by default and shown deprecation warning.
- Added support of minor `core-js` versions for simplify updating in the future.
- For preventing some order-related problems, polyfills in the both `core-js@3` plugins added on `post` stage in the order of `core-js-compat` data.
- Divided plugins and polyfills parts of `preset-env`, instead of 2 internal plugins for adding polyfills, we have 6: usage and entry versions of plugins for `core-js@2`, ### Current state:
`core-js@3`, `regenerator-runtime`.
- Added support `samsung` target (for Samsung Internet) since `core-js-compat` and `compat-table` now contains mapping for this, fixes#6602.
#### `useBuilIns: entry` with `corejs: 3`
- No longer transforms `@babel/polyfill`.
- Transforms **all possible** `core-js` entry points to import of related modules (based on data from [`core-js-compat`](https://unpkg.com/core-js-compat@3.0.0-beta.15/entries.json)).
- Since of this, we no longer need `shippedProposals` / `proposals` flags with `useBuilIns: entry`.
- Removes `regenerator-runtime/runtime` import where it's not required.
#### `useBuilIns: usage` with `corejs: 3`
- In addition to `shippedProposals`, added flag `proposals` (in `corejs: { version: 3, proposals: true }` format) for polyfill all proposals from `core-js`.
- Fixed list of dependencies in built-in definitions.
- Improved the way of determination method / built-in name and source of this method.
- Adds import of required polyfills on `MemberExpression`, `ObjectPattern`, `in` operator.
- Adds import of required polyfills on access to global object properties.
- Adds import of all required common iterators on all syntax features which use iterators protocol (`for-of`, destructuring, spread, `yield` delegation, etc.).
- Adds import of promises on syntax features which use promises (async functions/generators, dynamic import, etc.), fixes#9250, #7402, etc.
### `core-js@2` stuff
I didn't want to tough `core-js@2`-related stuff, however
- Fixed some serious errors in definitions which breaks `Object.getOwnPropertySymbols`, `Symbol.toStringTag` logic, `Promise#finally`, `Array#forEach`, etc.
- `Array#flatMap` and trim methods moved to stable features as a part of ES2019 and loaded by deprecated `@babel/polyfill` and `@babel/preset-env` with `corejs: 2` option.
