Make string juxtaposition combine properly with string interpolations.

frog/leg/ssa/nodes.dart

Index: frog/leg/ssa/nodes.dart
diff --git a/frog/leg/ssa/nodes.dart b/frog/leg/ssa/nodes.dart
index 9dd66314586ceb17236b364d272782a0cbdf1980..ed2e0945e316bd2a8425f9fef188fc91203ebae9 100644
--- a/frog/leg/ssa/nodes.dart
+++ b/frog/leg/ssa/nodes.dart
@@ -1406,7 +1406,7 @@ class HAdd extends HBinaryArithmetic {
                op2.isConstantNull());
         otherString = new DartString.literal(op2.constant.value.toString());
       }
-      DartString cons = new ConsDartString(leftString, otherString);
+      DartString cons = new DartString.cons(leftString, otherString);
       return graph.addConstantString(cons);
     }
     return super.fold(graph);