[js code hints] jump to definition doesn't jump to the file in another location out of JavaScriptCodeHints folder

[jodyzhang]

1. open "JavaScriptCodeHints" folder
2. open ScopeManager.js file
3. search "CollectionUtils.hasProperty", put cursor on "hasProperty", press command+j(to jump to hasProperty method
   -> no jump happens.
4. open CollectionUtils.js file(so it is loaded into brackets)
5. repeat step 3
   ->still no jump to that code

Expect: open the file(if not open), jump to that definition.

[pthiess]

Reviewed @dloverin -- case 3 seems fact of live - we may include working set files, however.

[peterflynn]

This has some commonality with #1377 -- I think both would be fixed by making FileIndexManager include working set files in its listings.

[redmunds]

Was assigned to tern team, so marking "Needs Review".