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

pkg/compiler/lib/src/js_backend/codegen/task.dart

Index: pkg/compiler/lib/src/js_backend/codegen/task.dart
diff --git a/pkg/compiler/lib/src/js_backend/codegen/task.dart b/pkg/compiler/lib/src/js_backend/codegen/task.dart
index edc7c55c375cf8cea9ebfadb0acf865055036e99..6ebc0dc8cd4c166f28458941764ab1d786e2ec01 100644
--- a/pkg/compiler/lib/src/js_backend/codegen/task.dart
+++ b/pkg/compiler/lib/src/js_backend/codegen/task.dart
@@ -216,9 +216,8 @@ class CpsFunctionCompiler implements FunctionCompiler {
     if (type is UnionTypeMask) {
       return '[${type.disjointMasks.map(formatTypeMask).join(', ')}]';
     } else if (type is FlatTypeMask) {
-      if (type.isEmpty) {
-        return "null";
-      }
+      if (type.isEmpty) return "empty";
+      if (type.isNull) return "null";
       String suffix = (type.isExact ? "" : "+") + (type.isNullable ? "?" : "!");
       return '${type.base.name}$suffix';
     } else if (type is ForwardingTypeMask) {