Use ConstExp for storing constants.

sdk/lib/_internal/compiler/implementation/helpers/stats.dart

Index: sdk/lib/_internal/compiler/implementation/helpers/stats.dart
diff --git a/sdk/lib/_internal/compiler/implementation/helpers/stats.dart b/sdk/lib/_internal/compiler/implementation/helpers/stats.dart
index 1ac3c73f017dd003584aecebb88b2f4161d9b44d..13897c72c46425d72160ed48b7f3e8b142204ce1 100644
--- a/sdk/lib/_internal/compiler/implementation/helpers/stats.dart
+++ b/sdk/lib/_internal/compiler/implementation/helpers/stats.dart
@@ -9,6 +9,7 @@ part of dart2js.helpers;
 /// Current stats collector. Use [enableStatsOutput] to enable recording of
 /// stats.
 Stats get stats {
+  enableDebugMode();
   if (_stats == null) {
     _stats = const Stats();
   }
@@ -34,6 +35,8 @@ void enableStatsOutput({CompilerOutputProvider outputProvider,
   if (_stats != null) {
     throw new StateError('Stats have already been initialized.');
   }
+  enableDebugMode();
+
   StatsOutput output;
   if (outputProvider != null) {
     if (extension == null) {