Remove DartString from constants.

pkg/compiler/lib/src/mirrors_used.dart

Index: pkg/compiler/lib/src/mirrors_used.dart
diff --git a/pkg/compiler/lib/src/mirrors_used.dart b/pkg/compiler/lib/src/mirrors_used.dart
index 70bc8acaf182ec7555760472edbe1b0a04e50649..aa86ad0b51687ac2c6214c74b2acd6b636f24547 100644
--- a/pkg/compiler/lib/src/mirrors_used.dart
+++ b/pkg/compiler/lib/src/mirrors_used.dart
@@ -383,7 +383,7 @@ class MirrorUsageBuilder {
       for (ConstantValue entry in list.entries) {
         if (entry.isString) {
           StringConstantValue string = entry;
-          result.add(string.primitiveValue.slowToString());
+          result.add(string.primitiveValue);
         } else if (!onlyStrings && entry.isType) {
           TypeConstantValue type = entry;
           result.add(type.representedType);
@@ -402,8 +402,7 @@ class MirrorUsageBuilder {
       return [type.representedType];
     } else if (constant.isString) {
       StringConstantValue string = constant;
-      var iterable =
-          string.primitiveValue.slowToString().split(',').map((e) => e.trim());
+      var iterable = string.primitiveValue.split(',').map((e) => e.trim());
       return onlyStrings ? new List<String>.from(iterable) : iterable.toList();
     } else {
       Spannable node = positionOf(constant);