Fix gen snapshot code to use Dart_LoadLibrary instead of Dart_LoadScript so that the root library value is not set when creating the bootstrap snapshot. The root library gets set when the application is loaded in later.
Fix gen snapshot code to use Dart_LoadLibrary instead of Dart_LoadScript so that the root library value is not set when creating the bootstrap snapshot. The root library gets set when the application is loaded in later.

R=johnmccutchan@google.com

Review URL: https://codereview.chromium.org/1653143002 .

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