Index: Lib/test/support.py
===================================================================
--- Lib/test/support.py	(revision 71863)
+++ Lib/test/support.py	(working copy)
@@ -516,30 +516,22 @@
     a context manager."""
 
     def __init__(self):
-        self._environ = os.environ
-        self._unset = set()
-        self._reset = dict()
+        self._environ = dict(os.environ)
 
     def set(self, envvar, value):
-        if envvar not in self._environ:
-            self._unset.add(envvar)
-        else:
-            self._reset[envvar] = self._environ[envvar]
-        self._environ[envvar] = value
+        os.environ[envvar] = value
 
     def unset(self, envvar):
-        if envvar in self._environ:
-            self._reset[envvar] = self._environ[envvar]
-            del self._environ[envvar]
+        del os.environ[envvar]
 
     def __enter__(self):
         return self
 
     def __exit__(self, *ignore_exc):
-        for envvar, value in self._reset.items():
-            self._environ[envvar] = value
-        for unset in self._unset:
-            del self._environ[unset]
+        for (k, v) in self._environ.items():
+            os.environ[k] = v
+        for k in set(os.environ) - set(self._environ):
+            del os.environ[k]
 
 class TransientResource(object):