All strong mode cleaning of dart2js.

pkg/compiler/lib/src/js_emitter/metadata_collector.dart

 // Copyright (c) 2013, 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.
 
 library dart2js.js_emitter.metadata_collector;
 
 import 'package:js_ast/src/precedence.dart' as js_precedence;
 
 import '../common.dart';
 import '../constants/values.dart';
@@ skipping 60 matching lines @@
     return _value;
   }
 
   bool get isUsed => _rc > 0;
 
   markSeen(jsAst.BaseVisitor visitor) {
     _rc++;
     if (_rc == 1) entry.accept(visitor);
   }
 
-  int compareTo(_MetadataEntry other) => other._rc - this._rc;
+  int compareTo(covariant _MetadataEntry other) => other._rc - this._rc;
 }
 
 abstract class Placeholder implements jsAst.DeferredNumber {
   bind(_MetadataEntry entry);
 }
 
 class _ForwardingMetadataEntry extends _MetadataEntry implements Placeholder {
   _MetadataEntry _forwardTo;
   var debug;
 
@@ skipping 379 matching lines @@
     if (token is _ForwardingMetadataEntry && !token.isBound) {
       _foundUnboundToken = true;
     }
   }
 
   bool findUnboundPlaceholders(jsAst.Node node) {
     node.accept(this);
     return _foundUnboundToken;
   }
 }