Support private class properties

Prettier 1.14.3 Playground link

--parser babylon

Input:

// @flow

class Test {
  #privateField: number;

  constructor() {
  this.#privateField = 1;
  }
}

Output:

SyntaxError: Unexpected token, expected ";" (4:16)
  2 | 
  3 | class Test {
> 4 |   #privateField: number;
    |                ^
  5 | 
  6 |   constructor() {
  7 |   this.#privateField = 1;

Expected behavior: It should format the code without any error.

[duailibe]

The second is because Flow AST is

ClassProperty {
  key: Identifier "privateField"
}

whereas the Babylon is

ClassProperty {
  key: PrivateName {
    id: Identifier "privateField"
  }
}

[lydell]

Interestingly, when using the Flow parser instead of Babylon the code parses and prints but generates invalid code:

Prettier 1.14.3 Playground link

--parser flow

Input:

// @flow

class Test {
  #privateField: number;

  constructor() {
  this.#privateField = 1;
  }
}

Output:

// @flow

class Test {
  privateField: number;

  constructor() {
    this.#privateField = 1;
  }
}

Second Output:

SyntaxError: Private fields must be declared before they can be referenced. `#privateField` has not been declared. (7:10)
   5 | 
   6 |   constructor() {
>  7 |     this.#privateField = 1;
     |          ^^^^^^^^^^^^^
   8 |   }
   9 | }
  10 |