diff --git a/Lib/importlib/test/extension/test_case_sensitivity.py b/Lib/importlib/test/extension/test_case_sensitivity.py
index 2ccec89..3865539 100644
--- a/Lib/importlib/test/extension/test_case_sensitivity.py
+++ b/Lib/importlib/test/extension/test_case_sensitivity.py
@@ -20,13 +20,13 @@
         with support.EnvironmentVarGuard() as env:
             env.unset('PYTHONCASEOK')
             loader = self.find_module()
-            self.assert_(loader is None)
+            self.assertIsNone(loader)
 
     def test_case_insensitivity(self):
         with support.EnvironmentVarGuard() as env:
             env.set('PYTHONCASEOK', '1')
             loader = self.find_module()
-            self.assert_(hasattr(loader, 'load_module'))
+            self.assertTrue(hasattr(loader, 'load_module'))
 
 
 
diff --git a/Lib/importlib/test/source/test_abc_loader.py b/Lib/importlib/test/source/test_abc_loader.py
index fc11e65..b95db04 100644
--- a/Lib/importlib/test/source/test_abc_loader.py
+++ b/Lib/importlib/test/source/test_abc_loader.py
@@ -157,10 +157,10 @@
         mock = self.mocker({name: path})
         with util.uncache(name):
             module = mock.load_module(name)
-            self.assert_(name in sys.modules)
+            self.assertIn(name, sys.modules)
         self.eq_attrs(module, __name__=name, __file__=path, __package__='pkg',
                         __loader__=mock)
-        self.assert_(not hasattr(module, '__path__'))
+        self.assertFalse(hasattr(module, '__path__'))
         return mock, name
 
     def test_module_reuse(self):
@@ -247,16 +247,16 @@
 
     mocker = PyPycLoaderMock
 
-    @source_util.writes_bytecode
+    @source_util.writes_bytecode_files
     def verify_bytecode(self, mock, name):
         assert name in mock.module_paths
-        self.assert_(name in mock.module_bytecode)
+        self.assertIn(name, mock.module_bytecode)
         magic = mock.module_bytecode[name][:4]
         self.assertEqual(magic, imp.get_magic())
         mtime = importlib._r_long(mock.module_bytecode[name][4:8])
         self.assertEqual(mtime, 1)
         bc = mock.module_bytecode[name][8:]
-
+        self.assertEqual(bc, mock.compile_bc(name))
 
     def test_module(self):
         mock, name = super().test_module()
@@ -286,7 +286,7 @@
     """Test that bytecode is properly handled based on
     sys.dont_write_bytecode."""
 
-    @source_util.writes_bytecode
+    @source_util.writes_bytecode_files
     def run_test(self, dont_write_bytecode):
         name = 'mod'
         mock = PyPycLoaderMock({name: os.path.join('path', 'to', 'mod')})
@@ -307,7 +307,7 @@
 
     """Test that bytecode is regenerated as expected."""
 
-    @source_util.writes_bytecode
+    @source_util.writes_bytecode_files
     def test_different_magic(self):
         # A different magic number should lead to new bytecode.
         name = 'mod'
@@ -323,7 +323,7 @@
         magic = mock.module_bytecode[name][:4]
         self.assertEqual(magic, imp.get_magic())
 
-    @source_util.writes_bytecode
+    @source_util.writes_bytecode_files
     def test_old_mtime(self):
         # Bytecode with an older mtime should be regenerated.
         name = 'mod'
diff --git a/Lib/importlib/test/source/test_case_sensitivity.py b/Lib/importlib/test/source/test_case_sensitivity.py
index 368e893..6fad881 100644
--- a/Lib/importlib/test/source/test_case_sensitivity.py
+++ b/Lib/importlib/test/source/test_case_sensitivity.py
@@ -36,18 +36,18 @@
         with test_support.EnvironmentVarGuard() as env:
             env.unset('PYTHONCASEOK')
             sensitive, insensitive = self.sensitivity_test()
-            self.assert_(hasattr(sensitive, 'load_module'))
-            self.assert_(self.name in sensitive._base_path)
-            self.assert_(insensitive is None)
+            self.assertTrue(hasattr(sensitive, 'load_module'))
+            self.assertIn(self.name, sensitive._base_path)
+            self.assertIsNone(insensitive)
 
     def test_insensitive(self):
         with test_support.EnvironmentVarGuard() as env:
             env.set('PYTHONCASEOK', '1')
             sensitive, insensitive = self.sensitivity_test()
-            self.assert_(hasattr(sensitive, 'load_module'))
-            self.assert_(self.name in sensitive._base_path)
-            self.assert_(hasattr(insensitive, 'load_module'))
-            self.assert_(self.name in insensitive._base_path)
+            self.assertTrue(hasattr(sensitive, 'load_module'))
+            self.assertIn(self.name, sensitive._base_path)
+            self.assertTrue(hasattr(insensitive, 'load_module'))
+            self.assertIn(self.name, insensitive._base_path)
 
 
 def test_main():
diff --git a/Lib/importlib/test/source/test_file_loader.py b/Lib/importlib/test/source/test_file_loader.py
index f3d75c3..d01a2a7 100644
--- a/Lib/importlib/test/source/test_file_loader.py
+++ b/Lib/importlib/test/source/test_file_loader.py
@@ -127,7 +127,7 @@
         self.assert_(module_name in sys.modules)
 
     # [bad magic]
-    @source_util.writes_bytecode
+    @source_util.writes_bytecode_files
     def test_bad_magic(self):
         with source_util.create_modules('_temp') as mapping:
             py_compile.compile(mapping['_temp'])
@@ -140,7 +140,7 @@
                 self.assertEqual(bytecode_file.read(4), imp.get_magic())
 
     # [bad timestamp]
-    @source_util.writes_bytecode
+    @source_util.writes_bytecode_files
     def test_bad_bytecode(self):
         zeros = b'\x00\x00\x00\x00'
         with source_util.create_modules('_temp') as mapping:
diff --git a/Lib/importlib/test/source/util.py b/Lib/importlib/test/source/util.py
index 280edb4..ca04edf 100644
--- a/Lib/importlib/test/source/util.py
+++ b/Lib/importlib/test/source/util.py
@@ -9,32 +9,23 @@
 from test import support
 
 
-def writes_bytecode(fxn):
-    """Decorator to protect sys.dont_write_bytecode from mutation."""
+def writes_bytecode_files(fxn):
+    """Decorator to protect sys.dont_write_bytecode from mutation and to skip
+    tests that require it to be set to False."""
+    if sys.dont_write_bytecode:
+        return lambda *args, **kwargs: None
     @functools.wraps(fxn)
     def wrapper(*args, **kwargs):
         original = sys.dont_write_bytecode
         sys.dont_write_bytecode = False
-        to_return = fxn(*args, **kwargs)
-        sys.dont_write_bytecode = original
+        try:
+            to_return = fxn(*args, **kwargs)
+        finally:
+            sys.dont_write_bytecode = original
         return to_return
     return wrapper
 
 
-def writes_bytecode_files(fxn):
-    """Decorator that returns the function if writing bytecode is enabled, else
-    a stub function that accepts anything and simply returns None."""
-    if sys.dont_write_bytecode:
-        return lambda *args, **kwargs: None
-    else:
-        @functools.wraps(fxn)
-        def wrapper(*args, **kwargs):
-            to_return = fxn(*args, **kwargs)
-            sys.dont_write_bytecode = False
-            return to_return
-        return wrapper
-
-
 def bytecode_path(source_path):
     for suffix, _, type_ in imp.get_suffixes():
         if type_ == imp.PY_COMPILED:
diff --git a/Lib/importlib/test/util.py b/Lib/importlib/test/util.py
index 9ff54a2..845e380 100644
--- a/Lib/importlib/test/util.py
+++ b/Lib/importlib/test/util.py
@@ -7,17 +7,18 @@
 
 
 def case_insensitive_tests(class_):
-    """Class decorator that nullifies tests that require a case-insensitive
+    """Class decorator that nullifies tests requiring a case-insensitive
     file system."""
-    if sys.platform not in ('win32', 'darwin', 'cygwin'):
-        original_name = os.listdir('.')[0]
-        if original_name.upper() != original_name:
-            changed_name = original_name.upper()
-        else:
-            changed_name = original_name.lower()
+    # Windows is the only OS that is *always* case-insensitive
+    # (OS X *can* be case-sensitive).
+    if sys.platform not in ('win32', 'cygwin'):
+        changed_name = __file__.upper()
+        if changed_name == __file__:
+            changed_name = __file__.lower()
         if os.path.exists(changed_name):
             return class_
-        return unittest.TestCase
+        else:
+            return unittest.TestCase
     else:
         return class_