Remove DartString from constants.

pkg/compiler/lib/src/deferred_load.dart

Index: pkg/compiler/lib/src/deferred_load.dart
diff --git a/pkg/compiler/lib/src/deferred_load.dart b/pkg/compiler/lib/src/deferred_load.dart
index 865894715b973668f15aed65d6f70ec8bf20605a..c9925d4e849eb57a1530397512d4868f5e80cf5a 100644
--- a/pkg/compiler/lib/src/deferred_load.dart
+++ b/pkg/compiler/lib/src/deferred_load.dart
@@ -1080,7 +1080,7 @@ class _DeclaredDeferredImport implements _DeferredImport {
         if (element == compiler.commonElements.deferredLibraryClass) {
           ConstructedConstantValue constant = value;
           StringConstantValue s = constant.fields.values.single;
-          result = s.primitiveValue.slowToString();
+          result = s.primitiveValue;
           break;
         }
       }