Remove DartString from constants.

tests/compiler/dart2js/deferred_follow_constant_dependencies_test.dart

Index: tests/compiler/dart2js/deferred_follow_constant_dependencies_test.dart
diff --git a/tests/compiler/dart2js/deferred_follow_constant_dependencies_test.dart b/tests/compiler/dart2js/deferred_follow_constant_dependencies_test.dart
index 4fd0b58f54ae6b1b9eb36614382b710afe1905ea..66bfd8245f23e33f55794eb21f09b7210f2fc0b7 100644
--- a/tests/compiler/dart2js/deferred_follow_constant_dependencies_test.dart
+++ b/tests/compiler/dart2js/deferred_follow_constant_dependencies_test.dart
@@ -32,8 +32,7 @@ void main() {
     backend.constants.compiledConstants.forEach(addConstantWithDependendencies);
     for (String stringValue in ["cA", "cB", "cC"]) {
       ConstantValue constant = allConstants.firstWhere((constant) {
-        return constant.isString &&
-            constant.primitiveValue.slowToString() == stringValue;
+        return constant.isString && constant.primitiveValue == stringValue;
       });
       Expect.notEquals(null, outputUnitForConstant(constant),
           "Constant value ${constant.toStructuredText()} has no output unit.");