[Stepping] variables bound with let/const not shown in Scopes when step out of function

[jasonLaster]

Steps to reproduce:

1. Open attached example.html in Firefox 53.0 or Firefox Developer Edition 54.0a2 (both affected, new and old debugger!).
2. Open Developer Tools, go to Debugger.
3. Set breakpoint in line 10 (first line in function).
4. Reload to hit breakpoint.
5. Click "step out".

Actual results:

• "Scopes" in debugger shows function scope with "someVar" variable, not a block scope with "someLet" and "someConst" variables.
• This seems inconsistent since execution after the "step out" pauses before leaving the function (and thus its scope): Why are let/const bound variables no longer visible, but var bound ones are?

Expected results:

• "Scopes" in debugger should also show block scope with "someLet" and "someConst" variables.
• This is also the behavior when setting a breakpoint at the closing brace and hitting that breakpoint (see screen capture video): "someLet", "someConst", and "someVar" are all visible as expected.

video: https://bug1362428.bmoattachments.org/attachment.cgi?id=8864898
from: https://bugzilla.mozilla.org/show_bug.cgi?id=1362428

[jasonLaster]

possibly related to #3390

[jasonLaster]

I'm closing this issue as it's a platform bug that we will fix in MC