Store toString value in enum.

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

 // Copyright (c) 2016, 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.
 
 import 'package:js_runtime/shared/embedded_names.dart';
 import 'package:kernel/ast.dart' as ir;
 
 import '../common.dart';
 import '../compiler.dart';
 import '../constants/expressions.dart';
@@ skipping 486 matching lines @@
     int index = _extractEnumIndexFromConstantValue(
         constant, _backend.helpers.jsBuiltinEnum);
     if (index == null) return null;
     return _backend.emitter.builtinTemplateFor(JsBuiltin.values[index]);
   }
 
   int _extractEnumIndexFromConstantValue(
       ConstantValue constant, ClassEntity classElement) {
     if (constant is ConstructedConstantValue) {
       if (constant.type.element == classElement) {
-        assert(constant.fields.length == 1);
-        ConstantValue indexConstant = constant.fields.values.single;
+        assert(constant.fields.length == 1 || constant.fields.length == 2);
+        ConstantValue indexConstant = constant.fields.values.first;
         if (indexConstant is IntConstantValue) {
           return indexConstant.primitiveValue;
         }
       }
     }
     return null;
   }
 
   ResolutionDartType getDartType(ir.DartType type) {
     return _typeConverter.convert(type);
@@ skipping 272 matching lines @@
       JumpTarget continueTarget =
           astAdapter.getJumpTarget(switchCase, isContinueTarget: true);
       assert(continueTarget is KernelJumpTarget);
       targetIndexMap[continueTarget] = switchIndex;
       assert(builder.jumpTargets[continueTarget] == null);
       builder.jumpTargets[continueTarget] = this;
       switchIndex++;
     }
   }
 }