Style - allow underline in identifier names

[zoechi]

I think the types, constant, and identifier name rules are too strict.

See dart-lang/linter#52

I think underlines should be allowed when the casing of the first character is correct and upper- and lower-case characters are used.

I think underlines should be allowed to avoid ambiguity.
This didn't matter as long as there was no linter but I think it's too inconvenient to add a linter ignore comment for each such name (not supported yet anyway)

[munificent]

Sorry, but I really don't think underscores need to be allowed in those kinds of names.