avoid substitution of global variables (#1557)

- unless `toplevel` is enabled
- global `const` works as before

lib/compress.js

@@ -223,6 +223,7 @@ merge(Compressor.prototype, {
 
     AST_Node.DEFMETHOD("reset_opt_flags", function(compressor, rescan){
         var reduce_vars = rescan && compressor.option("reduce_vars");
+        var toplevel = compressor.option("toplevel");
         var ie8 = !compressor.option("screw_ie8");
         var safe_ids = [];
         push();
@@ -334,7 +335,11 @@ merge(Compressor.prototype, {
         }
 
         function reset_def(def) {
-            def.fixed = undefined;
+            if (toplevel || !def.global || def.orig[0] instanceof AST_SymbolConst) {
+                def.fixed = undefined;
+            } else {
+                def.fixed = false;
+            }
             def.references = [];
             def.should_replace = undefined;
         }