message: tf2.enabled() should return False if disable_v2_behavior()
  is called, even if the TF2_BEHAVIOR env variable is set.
diff: @@ -25,7 +25,7 @@ from __future__ import print_function
  import os
  -_force_enable = False
  +_force_enable = None
  def enable():
  @@ -42,4 +42,7 @@ def disable():
  def enabled():
  """Returns True iff TensorFlow 2.0 behavior should be enabled."""
  - return _force_enable or os.getenv("TF2_BEHAVIOR", "0") != "0"
  + if _force_enable is None:
  + return os.getenv("TF2_BEHAVIOR", "0") != "0"
  + else:
  + return _force_enable
  