Make string juxtaposition combine properly with string interpolations.

frog/leg/tree/visitors.dart

Index: frog/leg/tree/visitors.dart
diff --git a/frog/leg/tree/visitors.dart b/frog/leg/tree/visitors.dart
index b0e96a4b6d5ff3654dd0500b3ac95e3284440fdd..f3e584d1ed99b0ec5ce5da9b7d92148a2c72800a 100644
--- a/frog/leg/tree/visitors.dart
+++ b/frog/leg/tree/visitors.dart
@@ -3,6 +3,8 @@
 // BSD-style license that can be found in the LICENSE file.
 
 class AbstractVisitor<R> implements Visitor<R> {
+  const AbstractVisitor();
+
   abstract R visitNode(Node node);
 
   R visitBlock(Block node) => visitStatement(node);
@@ -32,9 +34,7 @@ class AbstractVisitor<R> implements Visitor<R> {
   R visitLiteralMapEntry(LiteralMapEntry node) => visitNode(node);
   R visitLiteralNull(LiteralNull node) => visitLiteral(node);
   R visitLiteralString(LiteralString node) => visitLiteral(node);
-  R visitLiteralStringJuxtaposition(LiteralStringJuxtaposition node) {
-    return visitLiteralString(node);
-  }
+  R visitStringJuxtaposition(Juxtaposition node) => visitExpression(node);
   R visitLoop(Loop node) => visitStatement(node);
   R visitModifiers(Modifiers node) => visitNode(node);
   R visitNamedArgument(NamedArgument node) => visitExpression(node);