Expose DiagnosticGroups.forName(name) as a static method

[realityforge]

No description provided.

[blickly]

Internal issue created http://b/129796346

[brad4d]

@realityforge I don't have a problem merging this PR, but I presume that this is pre-work for solving some problem. Could you add some explanation here, hopefully with links, to explain why you want this change?

[realityforge]

I am away from my development computer atm so this may lack in specifics but essentially google/jsinterop-generator uses closure compiler programatically to read externs. If closure compiler detects an error it should fail but does not atm (See google/jsinterop-generator#28) and this resulted in some errors in downstream projects (i.e. google/elemental2 that went undetected (Mostly as elemental2 patches and selectively imports externs).

To fix this I need to enable new warnings to pick up problems (in this specific case it is both an unknown type error as well as an @override being present without method on parent type and probably more when I get into it)

Currently I have to do something like

    CompilerOptions options = new CompilerOptions();
     options.setWarningLevel(new DiagnosticGroups().forName( "checkTypes" ), CheckLevel.ERROR);

which is just cumbersome. Exposing the method as static seems reasonable as it is a static concept and the 51fd067#diff-657416cab37a79f6ee2356a5bb860d96L105 change does something similar to the getRegisteredGroups()

HTH

[brad4d]

@realityforge Thanks for the explanation.

PR imported and sent for internal testing and review.

[brad4d]

Submitted internally.
Should be in tomorrow's push to GitHub.