Make string juxtaposition combine properly with string interpolations.

frog/leg/tree/unparser.dart

Index: frog/leg/tree/unparser.dart
diff --git a/frog/leg/tree/unparser.dart b/frog/leg/tree/unparser.dart
index d4b12f148705a2cc0a9eca1e983088fcc518a6ab..5506ac1ee2de083f2bcc1b5529d3f924c12578e9 100644
--- a/frog/leg/tree/unparser.dart
+++ b/frog/leg/tree/unparser.dart
@@ -119,15 +119,10 @@ class Unparser implements Visitor {
     add(node.token.value);
   }
 
-  visitLiteralStringJuxtaposition(LiteralStringJuxtaposition node) {
-    for (Link<Expression> literals = node.literals;
-         !literals.isEmpty();
-         literals = literals.tail) {
-      visit(literals.head);
-      if (!literals.tail.isEmpty()) {
-        sb.add(" ");
-      }
-    }
+  visitStringJuxtaposition(LiteralStringJuxtaposition node) {
+    visit(node.first);
+    sb.add(" ");
+    visit(node.second);
   }
 
   visitLiteralNull(LiteralNull node) {