Clean the output of generator: Use /// comments and run dartfmt/dart_style on output.

[mkustermann]

• [x] New style-guide: Use /// comments
• [x] Run new dart_style/dartfmt on output
• [x] Change generated sdk constraint to be <2.0.0-dev.infinity.
• [ ] Remove unnecessary parenthesis from (a).toJson()
• [ ] New style-guide: Use camelCase constant naming (consider adding a flag)

[jakobr-google]

While here, update SDK constraint in emitted pubspec.yaml to <2.0.0-dev.infinity?

Third bullet is also #131.

[mkustermann]

We have to U-turn on the infinity sdk constraint after the new guidelines. PR is https://github.com/dart-lang/discoveryapis_generator/pull/175