All strong mode cleaning of dart2js.

pkg/compiler/lib/src/ordered_typeset.dart

Index: pkg/compiler/lib/src/ordered_typeset.dart
diff --git a/pkg/compiler/lib/src/ordered_typeset.dart b/pkg/compiler/lib/src/ordered_typeset.dart
index bbc71c9b08bc83d274adbb84a41ed2f017255370..b01dbf30e9a8b90911b000d71daae4a6c248a20f 100644
--- a/pkg/compiler/lib/src/ordered_typeset.dart
+++ b/pkg/compiler/lib/src/ordered_typeset.dart
@@ -171,10 +171,11 @@ abstract class OrderedTypeSetBuilderBase implements OrderedTypeSetBuilder {
   OrderedTypeSetBuilderBase(this.cls, {this.reporter, InterfaceType objectType})
       : this._objectType = objectType;
 
-  InterfaceType getThisType(ClassEntity cls);
-  InterfaceType substByContext(InterfaceType type, InterfaceType context);
-  int getHierarchyDepth(ClassEntity cls);
-  OrderedTypeSet getOrderedTypeSet(ClassEntity cls);
+  InterfaceType getThisType(covariant ClassEntity cls);
+  InterfaceType substByContext(
+      covariant InterfaceType type, covariant InterfaceType context);
+  int getHierarchyDepth(covariant ClassEntity cls);
+  OrderedTypeSet getOrderedTypeSet(covariant ClassEntity cls);
 
   OrderedTypeSet createOrderedTypeSet(
       InterfaceType supertype, Link<DartType> interfaces) {