Add uglify as a target

[yavorsky]

Add uglify as a target. Fixes #134.
I've not given attention to uglify's harmony brunch because of there no info about what ES6 features they currently support and how stable this support is (seems like not much for now).

Also, as for me, there is no difference for us between uglify and uglify 2. Both haven't es6 support at all. So allowed uglify values are numbers and booleans.

If uglify hasn't es6 support, what's the difference between no targets specifying at all?
The difference is in built-ins. For uglify no matter what polyfills you are using - it's just methods and there are no problems to parse them.

Ex:
.babelrc

{"presets":
  [
    ["env", {
      "targets": {
        "chrome": 55,
        "uglify": true
      },
      "useBuiltIns": true,
      "modules": false
    }]
  ]
}

Will use all the possible plugins and only built-ins based on specified targets (except uglify).

[existentialism]

Awesome!

[existentialism]

Just a couple grammar nits:

If you are using UglifyJS to minify your code, then targeting later browsers will throw a syntax error.

To prevent this - specify the uglify option, which will enable all plugins and as a result, fully compile your code to ES5. Note, that useBuiltIns will work as before, and only include the polyfills that your target(s) need.

[yavorsky]

@existentialism Oh, thanks!!! 😊

[existentialism]

@yavorsky minor, but I pushed a debug test when using uglify output (wanted to make sure it shows "uglify: true)

[hzoo]

If we didn't have to do this, would be good? doesn't seem necessary

[yavorsky]

It will allow null or false and use it exactly like a true. I think we must check other targets the same way.

[hzoo]

although currently we only support uglify2

[yavorsky]

@hzoo I think 1 also. No?

[hzoo]

I guess I just mean it's the same for whatever value

[felixfbecker]

This doesn't seem to enable transpilation of new.target - I get an UglifyJS error

Unexpected token: punc (.)

[hzoo]

@felixfbecker it wasn't supported at all in Babel until recently, and we didn't add it to the preset yet, and maybe you should try the newer uglify version?

[felixfbecker]

@hzoo Sorry, I assumed so because the PR was merged in July: babel/babel#5906
What would I need to do to get it working?

[hzoo]

It's actually because we never actually added it to the env/es2015 presets. I guess you could do it manually in plugins for now as a workaround

[felixfbecker]

Shouldn't it be included through the class transform?

The new.target property, is transpiled to this.constructor. As, with pure function constructor based prototype inheritance, the this instance is created by the child class, and given to the parent class constructor & methods (see super), this.constructor refers to the child class prototype constructor property, itself fixed to the child class function itself, matching to the constructor invoked by new.

https://www.npmjs.com/package/babel-plugin-transform-class#newtarget

Could I do a PR to add it?