Remove DartString from constants.

pkg/compiler/lib/src/ssa/optimize.dart

Index: pkg/compiler/lib/src/ssa/optimize.dart
diff --git a/pkg/compiler/lib/src/ssa/optimize.dart b/pkg/compiler/lib/src/ssa/optimize.dart
index 4436023b10b8f494823fe8d52977563e52062601..164f2dc884b7025adcca19e6afba5a7f3a5eb0a9 100644
--- a/pkg/compiler/lib/src/ssa/optimize.dart
+++ b/pkg/compiler/lib/src/ssa/optimize.dart
@@ -19,7 +19,6 @@ import '../js_backend/interceptor_data.dart' show InterceptorData;
 import '../js_backend/native_data.dart' show NativeData;
 import '../native/native.dart' as native;
 import '../options.dart';
-import '../tree/dartstring.dart' as ast;
 import '../types/types.dart';
 import '../universe/selector.dart' show Selector;
 import '../universe/side_effects.dart' show SideEffects;
@@ -1135,8 +1134,8 @@ class SsaInstructionSimplifier extends HBaseVisitor
     }
 
     HInstruction folded = _graph.addConstant(
-        constantSystem.createString(new ast.DartString.concat(
-            leftString.primitiveValue, rightString.primitiveValue)),
+        constantSystem.createString(
+            leftString.primitiveValue + rightString.primitiveValue),
         _closedWorld);
     if (prefix == null) return folded;
     return new HStringConcat(