Make string juxtaposition combine properly with string interpolations.

frog/leg/resolver.dart

Index: frog/leg/resolver.dart
diff --git a/frog/leg/resolver.dart b/frog/leg/resolver.dart
index 7cf4d9c4f55b7b9741004451716f387abf05eb60..b76af222d455526a97aafb8451f09a15719211bc 100644
--- a/frog/leg/resolver.dart
+++ b/frog/leg/resolver.dart
@@ -917,6 +917,10 @@ class ResolverVisitor extends CommonResolverVisitor<Element> {
   visitLiteralNull(LiteralNull node) {
   }
 
+  visitStringJuxtaposition(StringJuxtaposition node) {
+    if (node.isInterpolation) node.visitChildren(this);
+  }
+
   visitNodeList(NodeList node) {
     for (Link<Node> link = node.nodes; !link.isEmpty(); link = link.tail) {
       visit(link.head);