fix(benchpress): add index to root of module

This is necessary when using require('benchpress') in node.

modules/benchpress/index.dart

@@ -0,0 +1,2 @@
+library benchpress.index;
+//no dart implementation

modules/benchpress/index.ts

@@ -0,0 +1,2 @@
+require(require('traceur').RUNTIME_PATH);
+module.exports = require('./benchpress.js');