Store toString value in enum.

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

Index: pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
diff --git a/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart b/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
index c12d7c40d15f0cb2628afbce39387a613a1724da..56c69ee300b42bc3509c81be3fdba853761a82a0 100644
--- a/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
+++ b/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
@@ -504,8 +504,8 @@ class KernelAstAdapter extends KernelElementAdapterMixin {
       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;
         }