Fix webchannel-wrapper sourcemap. (#1590)

I accidentally broke this while tweaking my last change to write to a temp folder.

Author: mikelehen

packages/webchannel-wrapper/tools/build.js

@@ -68,7 +68,7 @@ closureBuilder.build(
     options: {
       closure: {
         output_wrapper:
-          '%output%\n//# sourceMappingURL=index.closure-es.js.map',
+          '%output%\n//# sourceMappingURL=index.js.map',
         language_out: 'ECMASCRIPT5',
         compilation_level: 'ADVANCED',
         define: closureDefines