parse @global_defs as expressions (#2169)

- let parser rejects non-conformant input
- eliminate need for extraneous parenthesis

lib/compress.js

@@ -92,12 +92,9 @@ function Compressor(options, false_by_default) {
     var global_defs = this.options["global_defs"];
     if (typeof global_defs == "object") for (var key in global_defs) {
         if (/^@/.test(key) && HOP(global_defs, key)) {
-            var ast = parse(global_defs[key]);
-            if (ast.body.length == 1 && ast.body[0] instanceof AST_SimpleStatement) {
-                global_defs[key.slice(1)] = ast.body[0].body;
-            } else throw new Error(string_template("Can't handle expression: {value}", {
-                value: global_defs[key]
-            }));
+            global_defs[key.slice(1)] = parse(global_defs[key], {
+                expression: true
+            });
         }
     }
     var pure_funcs = this.options["pure_funcs"];