Folding of multi-line comments above certain JavaScript ES6 constructions

[scriptfoundry]

• VSCode Version: 1.25.1
• OS Version: OS X 10.12.6

Steps to Reproduce:

1. Open VS Code
2. Create new file
3. Set language type to JavaScript
4. Enter the following example code:

    /**
     * Return a sum
     * @param y {Number}
     * @param z {Number}
     * @returns {Number} the sum of y and z
     */
    function sum1(y, z) {
        return y + z;
    }

    /**
     * Return a sum
     * @param y {Number}
     * @param z {Number}
     * @returns {Number} the sum of y and z
     */
    const sum2 = (y, z) => {
        return y + z;
    };

5. Attempt to fold each comment block. The first one folds. The second one has no folding mark and cannot fold.
6. Shut down VS Code without closing this file
7. Reopen VS Code
8. Folding marks will appear briefly before the second comment but it will not be clickable and will disappear after about a second. No comments appear in the developer's console.
9. Change the sum2 function to a normal (ES5) function declaration and the comment folding marks reappear and the comment folds normally.

Does this issue occur when all extensions are disabled?: Yes

Notes:

While searching for a way to duplicate, I found that this same multi-line-comment folding behaviour happened in other languages, including C, C# and PHP, but I could not find any way to replicate it in Java. I have no idea if this problem is actually present in these other languages.

[mjbvz]

Opened microsoft/TypeScript#26014 to track this upstream