Pass the --module argument to the SM shell in the expected location

Ref tc39/test262-harness#163

Author: Ms2ger

lib/agents/jsshell.js

@@ -11,13 +11,17 @@ const stackRe = /^([\s\S]*?)\r?\nStack:\r?\n([\s\S]*)$/;
 const stackFrameRe = /^(.*?)?@(.*?):(\d+):(\d+)?$/;
 
 class JSShell extends ConsoleAgent {
-  async evalScript(code, options = {}) {
-    if (options.module && this.args[0] !== '--module') {
-      this.args.unshift('--module');
+  constructor(options) {
+    super(options);
+    if (this.args.indexOf('--module') !== -1) {
+      throw new Error("Passing --module as a SpiderMonkey host argument is not supported.")
     }
+  }
 
-    if (!options.module && this.args[0] === '--module') {
-      this.args.shift();
+  async evalScript(code, options = {}) {
+    if (options.module) {
+      // The file name must come immediataly after the `--module` flag.
+      this.args.push('--module');
     }
 
     return super.evalScript(code, options);