Replace TypeMask.isEmpty by isEmptyOrNull, use isEmpty when it's really empty.

pkg/compiler/lib/src/ssa/types.dart

Index: pkg/compiler/lib/src/ssa/types.dart
diff --git a/pkg/compiler/lib/src/ssa/types.dart b/pkg/compiler/lib/src/ssa/types.dart
index a34e60b9a7e971e5a7809b53bf8ab4fd3b171a5f..530a9684596e2c76f375a0680b7ce731bf73bd06 100644
--- a/pkg/compiler/lib/src/ssa/types.dart
+++ b/pkg/compiler/lib/src/ssa/types.dart
@@ -48,7 +48,7 @@ class TypeMaskFactory {
     TypeMask result = nativeBehavior.typesReturned
         .map((type) => fromNativeType(type, compiler))
         .reduce((t1, t2) => t1.union(t2, classWorld));
-    assert(!(result.isEmpty && !result.isNullable));
+    assert(!result.isEmpty);
     return result;
   }