Warn on variables starting with underscores

[evmar]

To Closure, a variable name that starts with an underscore prevents optimization.
Tsickle should warn/error on code that uses an underscore.

[mprobst]

Oh great. Starting with an underscore is the convention for private fields in Dart, so tons of fields in Angular 2 start with an underscore.

[evmar]

I'm having difficulty coming up with a case with one of the Closure inspectors that demonstrates this problem, but I'm sure I've seen it. See also microsoft/TypeScript#7345

[rkirov]

I think leading underscore is the escape hatch for 'variable renaming', which is completely different from 'property renaming'. So class fields are save, but top-level functions like __extend are not.

[evmar]

I cannot repro varying behavior here, and we disallow this per style already.