Delete unreachable default clause. (#1547)

The Dart analyzer will soon be changed so that if the `default` clause
of a `switch` statement is determined to be unreachable by the
exhaustiveness checker, a new warning of type
`unreachable_switch_default` will be issued. This parallels the
behavior of the existing `unreachable_switch_case` warning, which is
issued whenever a `case` clause of a `switch` statement is determined
to be unreachable.

This PR deletes an unreachable `default` clause from `dart_style` now,
to avoid a spurious warning when the analyzer change lands.

lib/src/front_end/piece_factory.dart

@@ -837,8 +837,6 @@ mixin PieceFactory {
               tokenPiece(combinatorNode.keyword),
               for (var name in names) tokenPiece(name.token, commaAfter: true),
             ]));
-          default:
-            throw StateError('Unknown combinator type $combinatorNode.');
         }
       }