support string namespace in import & export (#5570)

lib/compress.js

@@ -294,7 +294,7 @@ Compressor.prototype.compress = function(node) {
         function export_symbol(sym) {
             if (!(sym instanceof AST_SymbolDeclaration)) return;
             var node = make_node(AST_SymbolExport, sym, sym);
-            node.alias = node.name;
+            node.alias = make_node(AST_String, node, { value: node.name });
             props.push(node);
         }
     });