Fix another set of build failures on Windows since r14116.

TBR=yangguo@chromium.org

Review URL: https://codereview.chromium.org/13465010

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14118 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/preparser.h

@@ -104,6 +104,11 @@ class DuplicateFinder {
 };
 
 
+#ifdef WIN32
+#undef Yield
+#endif
+
+
 class PreParser {
  public:
   enum PreParseResult {