Better mangling in frog for fixing parts of issue 1837.

frog/leg/ssa/js_names.dart

Index: frog/leg/ssa/js_names.dart
===================================================================
--- frog/leg/ssa/js_names.dart	(revision 4594)
+++ frog/leg/ssa/js_names.dart	(working copy)
@@ -173,8 +173,7 @@
    */
   static String getValid(String name) {
     if (reserved.contains(name)) {
-      name = '\$$name';
-      assert(!reserved.contains(name));
+      return getValid('\$$name');
     } else if (name.contains(@'$')) {
       name = name.replaceAll(@'$', @'$$');
     }