All strong mode cleaning of dart2js.

pkg/compiler/lib/src/constants/expressions.dart

 // Copyright (c) 2014, 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.constants.expressions;
 
 import '../common.dart';
 import '../constants/constant_system.dart';
 import '../common_elements.dart';
 import '../elements/entities.dart';
@@ skipping 80 matching lines @@
     return sb.toString();
   }
 
   /// Writes the structure of the constant into [sb].
   void _createStructuredText(StringBuffer sb);
 
   int _computeHashCode();
 
   int get hashCode => _hashCode ??= _computeHashCode();
 
-  bool _equals(ConstantExpression other);
+  bool _equals(covariant ConstantExpression other);
 
   bool operator ==(other) {
     if (identical(this, other)) return true;
     if (other is! ConstantExpression) return false;
     if (kind != other.kind) return false;
     if (hashCode != other.hashCode) return false;
     return _equals(other);
   }
 
   String toString() {
@@ skipping 1815 matching lines @@
     visit(exp.name);
     if (exp.defaultValue != null) {
       sb.write(', defaultValue: ');
       visit(exp.defaultValue);
     }
     sb.write(')');
   }
 
   String toString() => sb.toString();
 }