Refactoring of string literals. Implement static string addition.

frog/leg/scanner/listener.dart

Index: frog/leg/scanner/listener.dart
diff --git a/frog/leg/scanner/listener.dart b/frog/leg/scanner/listener.dart
index 5e23c72206f66e4322984903e4b5e351124e103f..a24bb5bc40479c1a0ac0330e086a109a34c33af5 100644
--- a/frog/leg/scanner/listener.dart
+++ b/frog/leg/scanner/listener.dart
@@ -724,7 +724,7 @@ class ElementListener extends Listener {
     bool isLast = true;
     for (int i = 0; i < count; i++) {
       LiteralString string = popNode();
-      QuotedString validation =
+      DartString validation =
           stringValidator.validateInterpolationPart(string.token, quoting,
                                                     isFirst: false,
                                                     isLast: isLast);
@@ -737,7 +737,7 @@ class ElementListener extends Listener {
     }
 
     LiteralString string = popNode();
-    QuotedString validation =
+    DartString validation =
         stringValidator.validateInterpolationPart(string.token, quoting,
                                                   isFirst: true,
                                                   isLast: isLast);