D:\src\jdk9-dev\build\windows-x86_64-normal-server-release\images\jdk\bin>.\jjs
  --language=es6
jjs> var obj = { foo() { print('hello') } }
jjs> obj.foo()
jdk.nashorn.internal.runtime.ParserException: <shell>:1:18 Expected ; but found
{
var obj = { foo() { print('hello') } }
                  ^

The following is fine though:

D:\src\jdk9-dev\build\windows-x86_64-normal-server-release\images\jdk\bin>.\jjs
  --language=es6 --lazy-compilation=false -ot=false
jjs> var obj = { foo() { print('hello') } }
jjs> obj.foo()
hello