fix optional params to mock methods, allow all signatures

lib/src/js_ast/printer.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of js_ast;
 
 
 class JavaScriptPrintingOptions {
   final bool shouldCompressOutput;
   final bool minifyLocalVariables;
@@ skipping 927 matching lines @@
     functionOut(namedFunction.function, namedFunction.name);
   }
 
   visitFun(Fun fun) {
     functionOut(fun, null);
   }
 
   visitArrowFun(ArrowFun fun) {
     localNamer.enterScope(fun);
     if (fun.params.length == 1 &&
-        (fun.params.single.type == null || !options.emitTypes)) {
+        fun.params[0] is Identifier &&
+        (!options.emitTypes || fun.params[0].type == null)) {
       visitNestedExpression(fun.params.single, SPREAD,
           newInForInit: false, newAtStatementBegin: false);
     } else {
       out("(");
       visitCommaSeparated(fun.params, SPREAD,
           newInForInit: false, newAtStatementBegin: false);
       out(")");
     }
     outTypeAnnotation(fun.returnType);
     spaceOut();
@@ skipping 740 matching lines @@
     declare(node.name);
     node.function.accept(this);
   }
 
   visitClassExpression(ClassExpression node) {
     declare(node.name);
     if (node.heritage != null) node.heritage.accept(this);
     for (Method element in node.methods) element.accept(this);
   }
 }