dart2js cps: Do not inline string interpolation helper.

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

Index: pkg/compiler/lib/src/cps_ir/type_mask_system.dart
diff --git a/pkg/compiler/lib/src/cps_ir/type_mask_system.dart b/pkg/compiler/lib/src/cps_ir/type_mask_system.dart
index ea8bb4300e7985ff8bc1c750966c86e92142f0e2..2042320f6fced3778e14b6c388c45cfde57b331c 100644
--- a/pkg/compiler/lib/src/cps_ir/type_mask_system.dart
+++ b/pkg/compiler/lib/src/cps_ir/type_mask_system.dart
@@ -450,6 +450,13 @@ class TypeMaskSystem implements AbstractValueDomain {
     return interceptedTypes.containsMask(t.nonNullable(), classWorld);
   }
 
+  @override
+  bool isDefinitelySelfInterceptor(TypeMask t, {bool allowNull: false}) {
+    assert(allowNull != null);
+    if (!allowNull && t.isNullable) return false;
+    return areDisjoint(t, interceptorType);
+  }
+
   /// Given a class from the interceptor hierarchy, returns a [TypeMask]
   /// matching all values with that interceptor (or a subtype thereof).
   @override