Make fasta analyzer happy with dart2js method type change.

pkg/compiler/lib/src/closure.dart

Index: pkg/compiler/lib/src/closure.dart
diff --git a/pkg/compiler/lib/src/closure.dart b/pkg/compiler/lib/src/closure.dart
index 5a94e8555e84cbaea0837a59250cff47b674f400..1fbd347baa0e03c6c46b53d82551970bb241dc90 100644
--- a/pkg/compiler/lib/src/closure.dart
+++ b/pkg/compiler/lib/src/closure.dart
@@ -53,7 +53,7 @@ abstract class ClosureDataLookup<T> {
 
   /// 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);