See #6658 for background.

Currently for JavaScript files we provide a very limited set of errors, mostly for syntax errors. There are a number of grammatical and type errors that would be useful to display also.

Brief notes from ad hoc discussion in the team room include:

• Move the code that flags TypeScript syntax in JavaScript files for better error handling
• Add a distinction between errors and warnings to avoid "valid" JavaScript containing errors
• Make the warnings user configurable as part of the project (i.e. may enable/disable certain warnings)

More detailed design to come...