Add a `strong-outputs` mode to build_sdk_summaries.dart.
Add a `strong-outputs` mode to build_sdk_summaries.dart.

This is needed by dev_compiler, which currently has its own fork of the
SDK that can only be summarized successfully in strong mode.

R=scheglov@google.com

Review URL: https://codereview.chromium.org/1979333002 .

dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk