Fix all strong mode warnings.

lib/src/version_union.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:collection/collection.dart';
 
 import 'utils.dart';
 import 'version.dart';
 import 'version_constraint.dart';
 import 'version_range.dart';
@@ skipping 37 matching lines @@
     }
 
     // Only allow Versions and VersionRanges here so we can more easily reason
     // about everything in [flattened]. _EmptyVersions and VersionUnions are
     // filtered out above.
     for (var constraint in flattened) {
       if (constraint is VersionRange) continue;
       throw new ArgumentError('Unknown VersionConstraint type $constraint.');
     }
 
-    (flattened as List).sort(compareMax);
+    flattened.sort(compareMax);
 
     var merged = <VersionRange>[];
     for (var constraint in flattened) {
       // Merge this constraint with the previous one, but only if they touch.
       if (merged.isEmpty ||
           (!merged.last.allowsAny(constraint) &&
               !areAdjacent(merged.last, constraint))) {
         merged.add(constraint);
       } else {
         merged[merged.length - 1] = merged.last.union(constraint);
@@ skipping 102 matching lines @@
 
   bool operator ==(other) {
     if (other is! VersionUnion) return false;
     return const ListEquality().equals(constraints, other.constraints);
   }
 
   int get hashCode => const ListEquality().hash(constraints);
 
   String toString() => constraints.join(" or ");
 }