DEV: updates js transpiler to use babel 7 (#10627)
Updates our js transpiler code to use Babel 7.11.6
List of changes in this commit:
- Updates plugins, babel plugins all have a new version which doesn't contain -es2015- anymore
- Drops [transform-es2015-classes](https://babeljs.io/docs/en/babel-plugin-transform-classes) this plugin shouldn't be needed now that we don't support IE
- Drops check-es2015-constants, checking constants is now part of babel and the check-constants plugin is deprecated. As a result the behavior slightly changed, and is now wrapping every const call in a readOnlyError function which would throw if assigned a new value. This explains the modified spec.
- Adds [proposal-optional-chaining](https://babeljs.io/docs/en/babel-plugin-proposal-optional-chaining)
```javascript
const obj = {
foo: {
bar: {
baz: 42,
},
},
};
const baz = obj?.foo?.bar?.baz; // 42
```
- Adds [proposal-json-strings](https://babeljs.io/docs/en/babel-plugin-proposal-json-strings)
```javascript
// IN
const ex = "before
after";
// ^ There's a U+2028 char between 'before' and 'after'
// OUT
const ex = "before\u2028after";
// ^ There's a U+2028 char between 'before' and 'after'
```
- Adds [proposal-nullish-coalescing-operator](https://babeljs.io/docs/en/babel-plugin-proposal-nullish-coalescing-operator)
```javascript
var object = {};
var foo = object.foo ?? "default"; // default
```
- Adds [proposal-logical-assignment-operators](https://babeljs.io/docs/en/babel-plugin-proposal-logical-assignment-operators)
```javascript
let a;
let b = 2;
a ||= b; // 2
```
- Adds [proposal-numeric-separator](https://babeljs.io/docs/en/babel-plugin-proposal-numeric-separator)
```javascript
let budget = 1_000_000_000_000;
console.log(budget === 10 ** 12); // true
```
- Adds proposal-object-rest-spread https://babeljs.io/docs/en/babel-plugin-proposal-object-rest-spread
```javascript
let { x, y, ...z } = { x: 1, y: 2, a: 3, b: 4 };
console.log(x); // 1
console.log(y); // 2
console.log(z); // { a: 3, b: 4 }
```
- Adds proposal-optional-catch-binding https://babeljs.io/docs/en/babel-plugin-proposal-optional-catch-binding
```javascript
try {
} catch {
} finally {
// ensures finally is available in every browsers
}
```
- Adds improved regex support for firefox through (transform-dotall-regex](https://babeljs.io/docs/en/next/babel-plugin-transform-dotall-regex.html) and (proposal-unicode-property-regex](https://babeljs.io/docs/en/babel-plugin-proposal-unicode-property-regex)
- Drops async/generator stuff, the browser we target should allow to use this (excepts iterable async)
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