fix dartdoc.bat

BUG=

Review URL: https://codereview.chromium.org//1318603006 .
diff --git a/sdk/bin/dartdoc.bat b/sdk/bin/dartdoc.bat
index 58c2017..c313e7f 100644
--- a/sdk/bin/dartdoc.bat
+++ b/sdk/bin/dartdoc.bat
@@ -14,7 +14,7 @@
 set DART=%BIN_DIR%\dart
 set SNAPSHOT=%BIN_DIR%\snapshots\dartdoc.dart.snapshot
 
-"%DART%" --packages="$BIN_DIR/snapshots/resources/dartdoc/.packages" "%SNAPSHOT%" %*
+"%DART%" --packages="%BIN_DIR%/snapshots/resources/dartdoc/.packages" "%SNAPSHOT%" %*
 
 endlocal