Make string juxtaposition combine properly with string interpolations.

frog/leg/typechecker.dart

Index: frog/leg/typechecker.dart
diff --git a/frog/leg/typechecker.dart b/frog/leg/typechecker.dart
index c3424c3f0c970031f54ef25e160cb291e4fb95db..ef555b2a420e88e1cba9224a75c35fbeffce3301 100644
--- a/frog/leg/typechecker.dart
+++ b/frog/leg/typechecker.dart
@@ -499,7 +499,9 @@ class TypeCheckerVisitor implements Visitor<Type> {
     return stringType;
   }
 
-  Type visitLiteralStringJuxtaposition(LiteralStringJuxtaposition node) {
+  Type visitStringJuxtaposition(StringJuxtaposition node) {
+    analyze(node.first);
+    analyze(node.second);
     return stringType;
   }