@static field should not be allowed to implement / override

``` scala
import annotation.static

abstract class A { def x: Int }

class T
object T extends A {
  @static override val x = 10
  def main(args: Array[String]): Unit = {
    println((this: A).x)
  }
}
```

compiles, gives `java.lang.AbstractMethodError: A.x()I`
