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

pkg/compiler/lib/src/js_backend/backend.dart

Index: pkg/compiler/lib/src/js_backend/backend.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend.dart b/pkg/compiler/lib/src/js_backend/backend.dart
index 2b10c540f799f430f2fcbd9f5238c04422795a18..ce403ad44d6edb0b5f439f11c65f0e5d20c06de4 100644
--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -1766,18 +1766,26 @@ class JavaScriptBackend extends Backend {
     return findHelper('assertSubtype');
   }
 
-  Element getCheckSubtypeOfRuntimeType() {
-    return findHelper('checkSubtypeOfRuntimeType');
+  Element getSubtypeCast() {
+    return findHelper('subtypeCast');
   }
 
-  Element getCheckDeferredIsLoaded() {
-    return findHelper('checkDeferredIsLoaded');
+  Element getCheckSubtypeOfRuntimeType() {
+    return findHelper('checkSubtypeOfRuntimeType');
   }
 
   Element getAssertSubtypeOfRuntimeType() {
     return findHelper('assertSubtypeOfRuntimeType');
   }
 
+  Element getSubtypeOfRuntimeTypeCast() {
+    return findHelper('subtypeOfRuntimeTypeCast');
+  }
+
+  Element getCheckDeferredIsLoaded() {
+    return findHelper('checkDeferredIsLoaded');
+  }
+
   Element getThrowNoSuchMethod() {
     return findHelper('throwNoSuchMethod');
   }