Use the before visitor in mangle props

(works around a bug in our tree walker which, while cloning nodes, breaks
references between labeled statements and break/continue labels)

lib/propmangle.js

@@ -102,7 +102,7 @@ function mangle_properties(ast, options) {
     }));
 
     // step 2: transform the tree, renaming properties
-    return ast.transform(new TreeTransformer(null, function(node){
+    return ast.transform(new TreeTransformer(function(node){
         if (node instanceof AST_ObjectKeyVal) {
             if (should_mangle(node.key)) {
                 node.key = mangle(node.key);