Can't get string literal completions for .js file if not --allowJs

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TypeScript Version: nightly (2.6.0-dev.20170928)

Code

a.d.ts

declare var x: { p: "a" | "b" };

b.js:

x = { /**/ }; // works, get completion for "p"
x.p = "/**/"; // don't get good completions

No tsconfig.json.

Expected behavior:

Get completions inside string literal in a .js file.

Actual behavior:

Get nothing.
If works if I add a tsconfig.json and set --allowJs.
If I enable // @ts-check, I correctly get an error if I use a wrong string literal, so we are picking up the types.
I'm not sure how to reproduce this error in a fourslash test, because apparently if I don't set // @allowJs: true, it can't find the b.js file. Here was my attempt:

/// <reference path="fourslash.ts" />

// @Filename: /a.d.ts
////declare var x: { p: "a" | "b" };

// @Filename: /b.js
////x = { p: "/**/" }

verify.completionsAt("", []);

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Turns out this isn't about allowJs and I was confused due to the property names being "a" and "b" and thus coming up first in alphabetical order.