dart2js cps: Type casts and related changes to type propagation.

pkg/compiler/lib/src/dart_types.dart

Index: pkg/compiler/lib/src/dart_types.dart
diff --git a/pkg/compiler/lib/src/dart_types.dart b/pkg/compiler/lib/src/dart_types.dart
index 18f9fa46afd2da5ab672db03ae5c1e513f11d469..47138acc990af0f9a59aabdd561ec0664b0e3974 100644
--- a/pkg/compiler/lib/src/dart_types.dart
+++ b/pkg/compiler/lib/src/dart_types.dart
@@ -1232,6 +1232,10 @@ abstract class DartTypes {
 
   /// Returns `true` if [t] is a subtype of [s].
   bool isSubtype(DartType t, DartType s);
+
+  /// Returns `true` if [t] might be a subtype of [s] for some values of
+  /// type variables in [s] and [t].
+  bool isPotentialSubtype(DartType t, DartType s);
 }
 
 class Types implements DartTypes {