Add native extensions for the Dart shell to the MacOs platform

runtime/bin/bin.gypi

Index: runtime/bin/bin.gypi
diff --git a/runtime/bin/bin.gypi b/runtime/bin/bin.gypi
index 27567b274ba43d328679bdc7082601e65528c2d4..6daac785fd48f24e2bdf076cea9a86ce66ef779a 100644
--- a/runtime/bin/bin.gypi
+++ b/runtime/bin/bin.gypi
@@ -419,11 +419,11 @@
     },
   ],
   'conditions': [
-    ['OS=="linux"', {
+    ['OS!="win"', {
       'targets': [
         {
           'target_name': 'test_extension',
-          'type': 'shared_library',
+          'type': 'loadable_module',
           'dependencies': [
           ],
           'include_dirs': [
@@ -431,10 +431,14 @@
           ],
           'sources': [
             'test_extension_linux.cc',
+            'test_extension_macos.cc',
           ],
           'defines': [
             'DART_SHARED_LIB',
           ],
+	  'xcode_settings': {
+ 	    'OTHER_LDFLAGS': [ '-undefined', 'dynamic_lookup' ],
+          },
         },
       ],
     }],