Summary:
Flow implemented class fields while they were very early-stage, behind the `esproposal.class_instance_fields` and `esproposal.class_static_fields` flowconfig options.

In current versions of the proposal, fields without initializers now default to `undefined`: [[design](https://github.com/tc39/proposal-class-fields#fields-without-initializers-are-set-to-undefined)] [[spec](https://tc39.es/proposal-class-fields/#sec-define-field)]

Babel < 8 stripped uninitialized fields, which is not spec-compliant. Babel 8 changes to leaving the field and just removing the annotation, as you'd rightly expect, but making it impossible* to have type-only fields. [[babel issue](babel/babel#10039)] [[babel 8 notes](babel/babel#10746)]

This diff introduces a `declare` syntax to provide a solution for type-only fields:

```
class C {
  declare foo: string;
  bar: string;
}

// becomes
class C {
  bar;
}
```

As far as Flow is concerned, both `foo` and `bar` are treated as before, as if they are `string`:

```
const x = new C;
(x.foo: string);
(x.bar: string);
```

Fixes #6811

[* technically you can use Flow comment syntax -- `class C { /*:: prop: string; */ }` but we don't want to require that]

Reviewed By: samwgoldman

Differential Revision: D20088452

fbshipit-source-id: f00299539c9387a55102e9a4e554a3b3bcb3498e

Summary:
Flow implemented class fields while they were very early-stage, behind the `esproposal.class_instance_fields` and `esproposal.class_static_fields` flowconfig options.

In current versions of the proposal, fields without initializers now default to `undefined`: [[design](https://github.com/tc39/proposal-class-fields#fields-without-initializers-are-set-to-undefined)] [[spec](https://tc39.es/proposal-class-fields/#sec-define-field)]

Babel < 8 stripped uninitialized fields, which is not spec-compliant. Babel 8 changes to leaving the field and just removing the annotation, as you'd rightly expect, but making it impossible* to have type-only fields. [[babel issue](babel/babel#10039)] [[babel 8 notes](babel/babel#10746)]

This diff introduces a `declare` syntax to provide a solution for type-only fields:

```
class C {
  declare foo: string;
  bar: string;
}

// becomes
class C {
  bar;
}
```

As far as Flow is concerned, both `foo` and `bar` are treated as before, as if they are `string`:

```
const x = new C;
(x.foo: string);
(x.bar: string);
```

Fixes #6811

[* technically you can use Flow comment syntax -- `class C { /*:: prop: string; */ }` but we don't want to require that]

Reviewed By: samwgoldman

Differential Revision: D20088452

fbshipit-source-id: f6102215d5cf13d91c073affd3adf85ce7d97a04