Can't use typescript declare to re-type derived class members

[rbabayoff]

Hey,

So when using typescript declare on a class member, I get the following unrelated flow error:

The 'declare' modifier is only allowed when the 'allowDeclareFields' option of @babel/plugin-transform-flow-strip-types or @babel/preset-flow is enabled.

Code to reproduce:

class Base {
  prop1: string | number = 0;
}

class Derived extends Base {
  declare prop1: string;
}