Make string juxtaposition combine properly with string interpolations.

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 c30e564c2da92366c3c4f02139bfac290c77a47e..02bbcfbbd663fbb1be2e3eb33c66a1d4571bf6a7 100644
--- a/frog/leg/scanner/listener.dart
+++ b/frog/leg/scanner/listener.dart
@@ -170,7 +170,7 @@ class Listener {
   void endLiteralString(int interpolationCount) {
   }
 
-  void handleLiteralStringJuxtaposition(int literalCount) {
+  void handleStringJuxtaposition(int literalCount) {
   }
 
   void beginMember(Token token) {
@@ -834,14 +834,16 @@ class ElementListener extends Listener {
     }
   }
 
-  void handleLiteralStringJuxtaposition(int literalCount) {
-    Link<Expression> literals = const EmptyLink<Expression>();
-    while (literalCount > 0) {
+  void handleStringJuxtaposition(int stringCount) {
+    assert(stringCount != 0);
+    Expression accumulator = popNode();
+    stringCount -= 1;
+    while (stringCount > 0) {
       Expression expression = popNode();
-      literals = literals.prepend(expression);
-      literalCount -= 1;
+      accumulator = new StringJuxtaposition(expression, accumulator);
+      stringCount -= 1;
     }
-    pushNode(new LiteralStringJuxtaposition(literals));
+    pushNode(accumulator);
   }
 }