Cleanup in localizations code (flutter#20018)

The following changes are made by this PR:

 * Translation bundles now implement MaterialLocalizations directly,
   and are public so that they can be directly extended.

 * The list of supported languages is now a generated constant.

 * The icuShortTimePattern/TimeOfDayFormat values are now pre-parsed.

 * Various other changes for consistency with the style guide and the
   rest of the codebase, e.g. the class names don't use `_`, the
   `path` library is imported as such, more dartdocs, fewer `//
   ignore`s, validation using exceptions.

This reduces our technical debt benchmark.

Author: Hixie

dev/tools/gen_date_localizations.dart

@@ -6,7 +6,8 @@
 /// package for the subset of locales supported by the flutter_localizations
 /// package.
 ///
-/// The extracted data is written into packages/flutter_localizations/lib/src/l10n/date_localizations.dart.
+/// The extracted data is written into:
+///   packages/flutter_localizations/lib/src/l10n/date_localizations.dart
 ///
 /// ## Usage
 ///
@@ -89,7 +90,7 @@ Future<Null> main(List<String> rawArgs) async {
   });
   buffer.writeln('};');
 
-  // Note: code that uses datePatterns expects it to contain values of type
+  // Code that uses datePatterns expects it to contain values of type
   // Map<String, String> not Map<String, dynamic>.
   buffer.writeln('const Map<String, Map<String, String>> datePatterns = const <String, Map<String, String>> {');
   patternFiles.forEach((String locale, File data) {