[ffigen] Typedefs for Dart types (#625) * WIP fix #386 * Fix tests * fmt * Update test expectations * Update test expectations * Fix analysis * fmt * Fix more tests * WIP refactor * Refactor `sameFfiDartAndCType` to not require a `Writer` * Fix tests * Fix analysis * Update test expectations * Format * Fix typedef_test.dart * Fix nits * Fix tests * Fmt * Regenerate more test files * Regen more tests * Remove accidental commit