refactor(change_detect): Make ChangeDetectionUtil#uninitialized a var

Previously, `uninitialized()` was a method, requiring a call as well as
two extra characters everywhere it was used.

Make this value a variable, saving the characters and avoiding the
method call to get its value.

This change also removes the export of `uninitialized` from
change_detect.ts, which is technically a breaking change, however
`uninitialized` is an implementation detail and nobody should be using
it in app logic. By convention, apps should not be importing from files
under `src/`.

Update to #3248.

modules/angular2/src/transform/template_compiler/change_detector_codegen.dart

@@ -256,7 +256,7 @@ class _CodegenState {
     var pipeType = r.name;
     return '''
       $_CURRENT_PROTO = $_PROTOS_ACCESSOR[$protoIndex];
-      if ($_IDENTICAL_CHECK_FN($pipe, $_UTIL.uninitialized())) {
+      if ($_IDENTICAL_CHECK_FN($pipe, $_UTIL.uninitialized)) {
         $pipe = $_PIPES_ACCESSOR.get('$pipeType', $context, $cdRef);
       } else if (!$pipe.supports($context)) {
         $pipe.onDestroy();