Use entities in BackendClasses

pkg/compiler/lib/src/constant_system_dart.dart

Index: pkg/compiler/lib/src/constant_system_dart.dart
diff --git a/pkg/compiler/lib/src/constant_system_dart.dart b/pkg/compiler/lib/src/constant_system_dart.dart
index 7c3ebc103d3ca7fffddf04f862ad9a8a1b0af6e8..f8aaaaf9eac7a3f815155397fcbb3b60eb1e6001 100644
--- a/pkg/compiler/lib/src/constant_system_dart.dart
+++ b/pkg/compiler/lib/src/constant_system_dart.dart
@@ -458,9 +458,8 @@ class DartConstantSystem extends ConstantSystem {
     // TODO(johnniwinther): Change the `Type` type to
     // `compiler.commonElements.typeType` and check the backend specific value
     // in [checkConstMapKeysDontOverrideEquals] in 'members.dart'.
-    ResolutionInterfaceType implementationType = compiler
-        .backend.backendClasses.typeImplementation
-        .computeType(compiler.resolution);
+    ResolutionInterfaceType implementationType =
+        compiler.backend.backendClasses.typeType;
     return new TypeConstantValue(type, implementationType);
   }