Remove DartString from constants.

pkg/compiler/lib/src/js_backend/namer.dart

Index: pkg/compiler/lib/src/js_backend/namer.dart
diff --git a/pkg/compiler/lib/src/js_backend/namer.dart b/pkg/compiler/lib/src/js_backend/namer.dart
index 01ce064a1ff3915a5e2194aa090bd3cc947a64e5..92f5b1427a05b0d5b92cd87cb667b2daf681be4c 100644
--- a/pkg/compiler/lib/src/js_backend/namer.dart
+++ b/pkg/compiler/lib/src/js_backend/namer.dart
@@ -1810,7 +1810,7 @@ class ConstantNamingVisitor implements ConstantValueVisitor {
   @override
   void visitString(StringConstantValue constant, [_]) {
     // No `addRoot` since string constants are always inlined.
-    addIdentifier(constant.primitiveValue.slowToString());
+    addIdentifier(constant.primitiveValue);
   }
 
   @override
@@ -1980,7 +1980,7 @@ class ConstantCanonicalHasher implements ConstantValueVisitor<int, Null> {
 
   @override
   int visitString(StringConstantValue constant, [_]) {
-    return _hashString(2, constant.primitiveValue.slowToString());
+    return _hashString(2, constant.primitiveValue);
   }
 
   @override