Property initialisation check in TS 3.5 is too strict for ember 3.5

Start reading here: https://github.com/microsoft/TypeScript/pull/29395#issuecomment-497376445

import Component from '@ember/component';
import defaultTo from 'lodash/default-to';

export default class Welcome extends Component {
  greeting: string = defaultTo(this.greeting, "Hello");
}

Briefly, ember 3.5's Components have properties that are initialised by the base to a value, but Typescript doesn't know about this. Users should be able to squash the error by writing greeting!: string = ..., or by turning off strictPropertyInitialization, but can't today.

Start with the linked TypeScript pull request and reproduce the Ember 3.5 example using the TypeScript 3.5 property-initialisation check. Compare the behavior of the shown declaration with the proposed greeting! form, then identify the relevant compiler entry point and regression coverage; done means the intended valid form no longer reports the error without weakening unrelated checks.