Make fasta analyzer happy with dart2js method type change.

pkg/compiler/lib/src/closure.dart

 // Copyright (c) 2012, 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 'common/names.dart' show Identifiers;
 import 'common/resolution.dart' show ParsingContext, Resolution;
 import 'common/tasks.dart' show CompilerTask, Measurer;
 import 'common.dart';
 import 'compiler.dart' show Compiler;
 import 'constants/expressions.dart';
@@ skipping 35 matching lines @@
   // one for local functions.
   ScopeInfo getScopeInfo(covariant Entity member);
 
   /// This returns the same information as ScopeInfo, but can be called in
   /// situations when you are sure you are dealing with a closure specifically.
   ClosureRepresentationInfo getClosureRepresentationInfo(
       covariant Entity member);
 
   /// Look up information about a loop, in case any variables it declares need
   /// to be boxed/snapshotted.
-  LoopClosureScope getLoopClosureScope(T loopNode);
+  LoopClosureScope getLoopClosureScope(covariant T loopNode);
 
   /// Accessor to the information about closures that the SSA builder will use.
   ClosureScope getClosureScope(MemberEntity entity);
 }
 
 /// Class that represents one level of scoping information, whether this scope
 /// is a closure or not. This is specifically used to store information
 /// about the usage of variables in try or sync blocks, because they need to be
 /// boxed.
 ///
@@ skipping 1473 matching lines @@
 ///
 /// Move the below classes to a JS model eventually.
 ///
 abstract class JSEntity implements MemberEntity {
   Local get declaredEntity;
 }
 
 abstract class PrivatelyNamedJSEntity implements JSEntity {
   Entity get rootOfScope;
 }