fix #27259, implement covariance checking for strong mode and DDC

pkg/dev_compiler/lib/src/compiler/reify_coercions.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:analyzer/analyzer.dart' as analyzer;
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_ast_factory.dart';
 import 'package:analyzer/dart/element/type.dart' show DartType;
 import 'package:analyzer/src/dart/ast/ast.dart' show FunctionBodyImpl;
 import 'package:analyzer/src/dart/ast/utilities.dart' show NodeReplacer;
@@ skipping 14 matching lines @@
 
   CoercionReifier._();
 
   /// Transforms the given compilation units, and returns a new AST with
   /// explicit coercion nodes in appropriate places.
   static List<CompilationUnit> reify(List<CompilationUnit> units) {
     var cr = new CoercionReifier._();
     return units.map(cr.visitCompilationUnit).toList(growable: false);
   }
 
-  /// Returns true if the `as` [node] was created by this class.
+  /// True if the `as` [node] is a required runtime check for soundness.
   // TODO(sra): Find a better way to recognize reified coercion, since we
   // can't set the isSynthetic attribute.
-  static bool isImplicitCast(AsExpression node) => node.asOperator.offset == 0;
+  static bool isRequiredForSoundness(AsExpression node) =>
+      node.asOperator.offset == 0;
 
   /// Creates an implicit cast for expression [e] to [toType].
   static Expression castExpression(Expression e, DartType toType) {
     // We use an empty name in the AST, because the JS code generator only cares
     // about the target type. It does not look at the AST name.
     var typeName =
         astFactory.typeName(AstBuilder.identifierFromString(''), null);
     typeName.type = toType;
     var cast = AstBuilder.asExpression(e, typeName);
     cast.staticType = toType;
@@ skipping 76 matching lines @@
     ResolutionCopier.copyResolutionData(node, copy);
     return copy;
   }
 }
 
 class _TreeCloner extends analyzer.AstCloner {
   void _cloneProperties(AstNode clone, AstNode node) {
     if (clone is Expression) {
       ast_properties.setImplicitCast(
           clone, ast_properties.getImplicitCast(node));
+      ast_properties.setImplicitOperationCast(
+          clone, ast_properties.getImplicitOperationCast(node));
       ast_properties.setIsDynamicInvoke(
           clone, ast_properties.isDynamicInvoke(node));
     }
+    if (clone is ClassDeclaration) {
+      ast_properties.setClassCovariantParameters(
+          clone, ast_properties.getClassCovariantParameters(node));
+      ast_properties.setSuperclassCovariantParameters(
+          clone, ast_properties.getSuperclassCovariantParameters(node));
+    }
   }
 
   @override
   /*=E*/ cloneNode/*<E extends AstNode>*/(/*=E*/ node) {
     var clone = super.cloneNode(node);
     _cloneProperties(clone, node);
     return clone;
   }
 
   @override
@@ skipping 25 matching lines @@
 
   // TODO(jmesserly): workaround for
   // https://github.com/dart-lang/sdk/issues/26368
   @override
   TypeName visitTypeName(TypeName node) {
     var clone = super.visitTypeName(node);
     clone.type = node.type;
     return clone;
   }
 }