fix #27353, support `@checked` covariant parameters in DDC

pkg/dev_compiler/lib/src/compiler/code_generator.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 'dart:collection' show HashMap, HashSet;
 import 'dart:math' show min, max;
 
 import 'package:analyzer/analyzer.dart' hide ConstantEvaluator;
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart' show Token, TokenType;
@@ skipping 2161 matching lines @@
           ]));
         } else if (param.kind == ParameterKind.POSITIONAL) {
           body.add(js.statement('if (# === void 0) # = #;',
               [jsParam, jsParam, _defaultParamValue(param)]));
         }
       }
 
       // TODO(jmesserly): various problems here, see:
       // https://github.com/dart-lang/dev_compiler/issues/116
       var paramType = param.element.type;
-      if (node is MethodDeclaration && _unsoundCovariant(paramType, true)) {
+      if (node is MethodDeclaration &&
+          (param.element.isCovariant || _unsoundCovariant(paramType, true)) &&
+          !_inWhitelistCode(node)) {
         var castType = _emitType(paramType,
             nameType: options.nameTypeTests || options.hoistTypeTests,
             hoistType: options.hoistTypeTests);
-        if (!_inWhitelistCode(node)) {
-          body.add(js.statement('#._check(#);', [castType, jsParam]));
-        }
+        body.add(js.statement('#._check(#);', [castType, jsParam]));
       }
     }
     return body.isEmpty ? null : _statement(body);
   }
 
   /// Given a type [t], return whether or not t is unsoundly covariant.
   /// If [contravariant] is true, then t appears in a contravariant
   /// position.
   bool _unsoundCovariant(DartType t, bool contravariant) {
     if (t is TypeParameterType) {
@@ skipping 3252 matching lines @@
 }
 
 bool isLibraryPrefix(Expression node) =>
     node is SimpleIdentifier && node.staticElement is PrefixElement;
 
 LibraryElement _getLibrary(AnalysisContext c, String uri) =>
     c.computeLibraryElement(c.sourceFactory.forUri(uri));
 
 bool _isDartRuntime(LibraryElement l) =>
     l.isInSdk && l.source.uri.toString() == 'dart:_runtime';