Build Optimizer does not support es2015 code

[filipesilva]

Build Optimizer does not currently support es2015 modules code, but we should support it.

Related to #237

[robwormald]

Should also support ES2015 code (really, it should support esnext)

[hansl]

I was under the impression UglifyES (or whatever NG version of uglify) was doing a proper job at DCE on ES2015 code and that we wouldn't need build optimizer for those.

[cyrilletuzi]

Note: now we have Angular Elements, support for ES2015 becomes more important, because transpiling custom elements to ES5 is not supported by the standard and produces an error.

[rafaelfgx]

With all the respect, this is a very basic bug and one that should be avoided.

You create an Angular 6 application, without changing anything, with all the default settings, build it for production, and it does not work.

It is very serious that something so trivial as publishing in production is not working in the final version.

More testing is needed, especially for the most important and essential tasks.

[robwormald]

@rafaelfgx - nobody should be using es2015 modules in production, Webpack doesn’t export them, and they’re not turned on by default.

[msklvsk]

While buildOptimizer is turned on by default, es6 is not, including the docs:

(angular.io/guide/typescript-configuration)

[devoto13]

@hansl It does not seem to be the case.

Here is ES2015 with build optimizer on (fails with angular/angular-cli#10658):

Here is ES2015 with build optimizer off:

As you can see it is ~200k less with build optimizer enabled. Not sure if it is real gain or a side effect of build optimizer removing not dead code, but this looks like a lot of gain. Most of it comes from Angular packages (~180k).