Bump the sdk-dependency to >=1.12.0
Bump the sdk-dependency to >=1.12.0

This is required because we use null-aware operators.
This fixes https://github.com/dart-lang/reflectable/issues/31

R=eernst@google.com

Review URL: https://codereview.chromium.org/1347803005 .

google/reflectable.dart,google/reflectable.dart,luisvt/reflectable,luisvt/reflectable