Remove magicness around nullMethod.

(1) Removes JSNI syntax @null::nullMethod and @null::nullField.
(2) Declares a noop function JCHSU.emptyFunction() that is used
    in the places where null.nullMethod() was used for that
    purpose, e.g. to make clinits idempotent.
(3) Declares an typeMarker function in JCHSU.typeMarker() to be used
     where the null.nullMethod was used as a typeMarker.
(4) Moves the setting of typeMarker at Object from code generation at
    GenerateJavaScriptAst to intrinsic in library JCHSU.defineClass.

Change-Id: Ifc309a782a2fc2af675e4645adb9cee4e95a1a40
10 files changed