fix(prod): fix function name mangling

Changed mangle configuration to mangle: { screw_ie8 : true, keep_fnames: true } to fix production builds issue - see #1644

This fixes issue [https://github.com/angular/angular-cli/issues/1644](https://github.com/angular/angular-cli/issues/1644)

Fix #1644
Close #1662

Author: abner

addon/ng2/models/webpack-build-production.ts

@@ -21,7 +21,7 @@ export const getWebpackProdConfigPartial = function(projectRoot: string, sourceD
       // ~107kb
       new webpack.optimize.UglifyJsPlugin({
         beautify: false, //prod
-        mangle: { screw_ie8 : true }, //prod
+        mangle: { screw_ie8 : true, keep_fnames: true }, //prod
         compress: { screw_ie8: true }, //prod
         comments: false //prod
       }),