Angular-cli does not compile vendor dependencies

[MatissJanis]

🐞 Bug report

Command (mark with an x)

- [ ] new
- [x] build
- [ ] serve
- [ ] test
- [ ] e2e
- [ ] generate
- [ ] add
- [ ] update
- [ ] lint
- [ ] xi18n
- [ ] run
- [ ] config
- [ ] help
- [ ] version
- [ ] doc

Is this a regression?

Not sure.

Description

When doing ng build on a project that is using a dependency that's compiled with es6, the output contains es6 code which makes older browsers break. See example below to understand this better.

🔬 Minimal Reproduction

1. Add a dependency that is NOT bundled to es5 (for example messageformat): https://github.com/MatissJanis/angular-cli-issue/commit/a406b57b5e3c40b8360a269c64177d32edc5ae86
2. Do ng build or ng build --prod
3. Check source code and you will notice template literal syntax here: https://github.com/MatissJanis/angular-cli-issue/blob/master/dist/test-ang/vendor.js

(the file is really large, please search for ${fn}(${args.join(', ')}) to see the issue)

Here's example screenshot of something that should NOT be in the build (since it makes IE11 and the like break).

🔥 Exception or Error

🌍 Your Environment

     _                      _                 ____ _     ___
    / \   _ __   __ _ _   _| | __ _ _ __     / ___| |   |_ _|
   / △ \ | '_ \ / _` | | | | |/ _` | '__|   | |   | |    | |
  / ___ \| | | | (_| | |_| | | (_| | |      | |___| |___ | |
 /_/   \_\_| |_|\__, |\__,_|_|\__,_|_|       \____|_____|___|
                |___/


Angular CLI: 6.2.9
Node: 12.1.0
OS: darwin x64
Angular: 6.1.10
... animations, common, compiler, compiler-cli, core, forms
... http, language-service, platform-browser
... platform-browser-dynamic, router

Package                           Version
-----------------------------------------------------------
@angular-devkit/architect         0.8.9
@angular-devkit/build-angular     0.8.9
@angular-devkit/build-optimizer   0.8.9
@angular-devkit/build-webpack     0.8.9
@angular-devkit/core              0.8.9
@angular-devkit/schematics        0.8.9
@angular/cli                      6.2.9
@ngtools/webpack                  6.2.9
@schematics/angular               0.8.9
@schematics/update                0.8.9
rxjs                              6.2.2
typescript                        2.9.2
webpack                           4.16.4

Anything else relevant?

[alan-agius4]

Hi, Angular-CLI doesn't down-level 3rd party JavaScript.

Libraries that are meant to support older browser versions are meant to ship both ES2015 and ES5 code. And it will be up to the build system to determine which on to use.

See https://docs.google.com/document/d/1CZC2rcpxffTDfRDs6p1cfbmKNLA6x5O-NtkJglDaBVs/preview#heading=h.k0mh3o8u5hx for more details.

This is very similar to #12975 (comment), insofar as we do not error if ES6 libraries are used when targetting ES5.

I think #12975 should be the main issue where third party libraries in ES2015 are discussed instead of having a fragmented discussion across multiple issues.

Thanks.

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