62367 Make dartdoc warnings fatal (& cleanup of warnings) (a: accessibility, a: internationalization, a: tests, cla: yes, f: cupertino, f: material design, framework, team, waiting for tree to go green)

62395 [gen_l10n] Synthetic package generation by default (a: internationalization, cla: yes, severe: API break, team, tool, waiting for tree to go green)

62582 Random unresolved doc reference fixes I missed before (a: internationalization, a: tests, cla: yes, f: material design, framework, waiting for tree to go green)

62755 Add punctuation for unterminated sentences in doc comments (a: accessibility, a: internationalization, a: tests, cla: yes, d: api docs, documentation, f: cupertino, f: material design, framework, tool)