StrictPropertyInitialization does not understand calls to helpers #21132

TypeScript Version: 2.7.0-dev.20180110

Code

export class Test {
    private _foo: string;
    constructor() {
        this._init();
    }

    private _init() {
        this._foo = 'test';
    }
}

https://github.com/berickson1/Playground/blob/master/strictPropertyInitError.ts

Expected behavior:
Code compiles with strict flags on

Actual behavior:
strictPropertyInitError.ts(4,13): error TS2564: Property '_foo' has no initializer and is not definitely assigned in the constructor.