Fix failure of test_compiler.py when compiling test_contextlib.py.
The culprit was an expression-less yield -- the first apparently in
the standard library.  I added a unit test for this.
Also removed the hack to force compilation of test_with.py.
diff --git a/Lib/compiler/transformer.py b/Lib/compiler/transformer.py
index eed9ce9..800461c 100644
--- a/Lib/compiler/transformer.py
+++ b/Lib/compiler/transformer.py
@@ -408,11 +408,11 @@
         return Discard(expr, lineno=expr.lineno)
 
     def yield_expr(self, nodelist):
-        if len(nodelist)>1:
-            value = nodelist[1]
+        if len(nodelist) > 1:
+            value = self.com_node(nodelist[1])
         else:
             value = Const(None)
-        return Yield(self.com_node(value), lineno=nodelist[0][2])
+        return Yield(value, lineno=nodelist[0][2])
 
     def raise_stmt(self, nodelist):
         # raise: [test [',' test [',' test]]]