Attempt to fix open source Yoga build.

Reviewed By: emilsjolander

Differential Revision: D4557733

fbshipit-source-id: b99701e9c1ec56fb147aa4cc738549df90aa7016

YOGA_DEFS

@@ -1,3 +1,7 @@
+with allow_unsafe_import():
+  import os.path
+def isdir(filename):
+  return os.path.isdir(filename)
 
 YOGA_ROOT = '//...'
 JAVA_TARGET = '//java:java'

csharp/BUCK

@@ -35,7 +35,7 @@ cxx_library(
   visibility = ['PUBLIC'],
 )
 
-if py_sdk.os.path.isdir('/Applications/Xcode.app'):
+if isdir('/Applications/Xcode.app'):
   yoganet_ios_srcs = []
   for arch in [
     'iphonesimulator-x86_64', 'iphonesimulator-i386', 'iphoneos-arm64', 'iphoneos-armv7'