strictPropertyInitialization with subclasses that refine the types of properties

TypeScript Version: 2.7.0-dev.20171226

Code

class A {
  x: {y: string} = {y: 'y'};
}

class B extends A {
  x: {y: string, z?: number};
}

Expected behavior:
I expected this not to warn because A#x is assignable to B#x.

Actual behavior:

index.ts(6,3): error TS2564: Property 'x' has no initializer and is not definitely assigned in the constructor.

Start by reproducing the minimal subclass example with strictPropertyInitialization enabled using the TypeScript compiler. Trace the type-checking path that reports TS2564 for the refined property, then add a regression test showing the diagnostic is omitted when the base initializer satisfies the subtype; run the relevant compiler tests to confirm existing initialization checks still pass.