dart2js cps: Do not mark result of type casts as non-null.

pkg/compiler/lib/src/cps_ir/type_propagation.dart

Index: pkg/compiler/lib/src/cps_ir/type_propagation.dart
diff --git a/pkg/compiler/lib/src/cps_ir/type_propagation.dart b/pkg/compiler/lib/src/cps_ir/type_propagation.dart
index f45de41c118b18a988863d9b17a9cfa16c14ee66..73282af04372d1fb61cdc611bd1c6ecfab33e25f 100644
--- a/pkg/compiler/lib/src/cps_ir/type_propagation.dart
+++ b/pkg/compiler/lib/src/cps_ir/type_propagation.dart
@@ -2769,7 +2769,7 @@ class TypePropagationVisitor implements Visitor {
         setReachable(cont);
         // Narrow type of output to those that survive the cast.
         TypeMask type = input.type.intersection(
-            typeSystem.subtypesOf(node.dartType),
+            typeSystem.subtypesOf(node.dartType).nullable(),
             classWorld);
         setValue(cont.parameters.single, nonConstant(type));
         break;