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

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

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// Generate code using the cps-based IR pipeline.
 library code_generator_task;
 
 import 'glue.dart';
 import 'codegen.dart';
 import 'unsugar.dart';
@@ skipping 198 matching lines @@
     inliner.cache.putUnoptimized(element, cpsFunction);
     return cpsFunction;
   }
 
   static const Pattern PRINT_TYPED_IR_FILTER = null;
 
   String formatTypeMask(TypeMask type) {
     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) {
       return formatTypeMask(type.forwardTo);
     }
     throw 'unsupported: $type';
   }
 
   void dumpTypedIr(String passName, cps.FunctionDefinition cpsFunction) {
     if (PRINT_TYPED_IR_FILTER != null &&
@@ skipping 120 matching lines @@
         treeOptimizationTask]
       ..addAll(fallbackCompiler.tasks);
   }
 
   js.Node attachPosition(js.Node node, AstElement element) {
     return node.withSourceInformation(
         sourceInformationFactory.createBuilderForContext(element)
             .buildDeclaration(element));
   }
 }