--allowJs (3.7.0 regression): Cannot reference this when extending imported function prototype

[Knagis]

TypeScript Version: 3.7.x-dev.20191021

Search Terms:

• Cannot set property 'symbol' of undefined

Code
Originally encountered when transpiling BabylonJS ES6 code (@babylonjs/core/Animations/animatable.js)

Minimum repro:

import { A } from "./a";

A.prototype.foo = function () {
    this.bar = 0;
};

Expected behavior:
Code is transpiled.

Actual behavior:

node_modules\typescript\lib\tsc.js:23641
            node.symbol = symbol;
                        ^

TypeError: Cannot set property 'symbol' of undefined
    at addDeclarationToSymbol (node_modules\typescript\lib\tsc.js:23641:25)
    at bindThisPropertyAssignment (node_modules\typescript\lib\tsc.js:25564:25)
    at bindWorker (node_modules\typescript\lib\tsc.js:25270:29)
    at bind (node_modules\typescript\lib\tsc.js:25165:13)
    at bindExpressionStatement (node_modules\typescript\lib\tsc.js:24496:13)
    at bindChildrenWorker (node_modules\typescript\lib\tsc.js:24002:21)
    at bindChildren (node_modules\typescript\lib\tsc.js:23923:17)
    at bind (node_modules\typescript\lib\tsc.js:25171:21)
    at node_modules\typescript\lib\tsc.js:23929:68
    at bindEach (node_modules\typescript\lib\tsc.js:23945:21)
error Command failed with exit code 1.

Playground Link:

http://www.typescriptlang.org/play/?ts=3.7-Beta&ssl=3&ssc=2&pln=1&pc=1#code/JYWwDg9gTgLgBAbzgQTgXzgMyhEcBEAdAPQCG+A3AFBXKFg4wQwCeYApoZhBHALxYArgDsAxjGARhcABQBKRFTjK4MABbAAzoQBGpKPzgAGamgpA (see errors in browser console)

Related Issues:

#34493

[Knagis]

@sandersn - you seem to be the person looking into the similar issues.

[Knagis]

Created a PR #34650 with a fix that follows the same approach as #34566.