dart2js: Support new const operations on strings.

pkg/compiler/lib/src/constant_system_dart.dart

Index: pkg/compiler/lib/src/constant_system_dart.dart
diff --git a/pkg/compiler/lib/src/constant_system_dart.dart b/pkg/compiler/lib/src/constant_system_dart.dart
index 30fb62ed9186249bdd42bb8be687cb00fb789c24..0c507a7037e2230f9d114736c9612bf04e9878b8 100644
--- a/pkg/compiler/lib/src/constant_system_dart.dart
+++ b/pkg/compiler/lib/src/constant_system_dart.dart
@@ -233,6 +233,12 @@ class AddOperation implements BinaryOperation {
       NumConstantValue rightNum = right;
       double result = leftNum.primitiveValue + rightNum.primitiveValue;
       return DART_CONSTANT_SYSTEM.createDouble(result);
+    } else if (left.isString && right.isString) {
+      StringConstantValue leftString = left;
+      StringConstantValue rightString = right;
+      DartString result = new DartString.concat(leftString.primitiveValue,
+                                                rightString.primitiveValue);
+      return DART_CONSTANT_SYSTEM.createString(result);
     } else {
       return null;
     }