Revive the Scala.js backend.

This partially reverts 06a3d47 to bring
back Dotty.js from the dead.

- Scala.js was upgraded to 1.0.0-M6, and the back-end was updated
  to reflect changes that have happened in the Scala 2 back-end
  since the initial implementation of the Dotty.js back-end.
- The backend was initially removed because of #1574, I don't see why
  GetClass has a requirement on FunctionalInterfaces, so I just removed
  it ¯\_(ツ)_/¯
- The Scala.js backend explodes when extending a Scala 2 trait
  because of the way LinkScala2Impls transforms supercalls to
  these traits. Luckily, we don't need to extend js.JSApp anymore
  to make a Hello World :P.

The following command works, and is added to the CI:

    sbt sjsSandbox/run

This generally remains a WIP. There are still a lot of
Scala.js-specific features which are not supported, notably exports
and non-native JS classes.

.drone.yml

@@ -37,6 +37,13 @@ pipeline:
       - ./project/scripts/sbt ";dotty-bootstrapped/compile ;dotty-bootstrapped/test"
       - ./project/scripts/bootstrapCmdTests
 
+  test_sjs:
+    group: test
+    image: lampepfl/dotty:2018-10-01
+    commands:
+      - cp -R . /tmp/3/ && cd /tmp/3/
+      - ./project/scripts/sbt ";sjsSandbox/run"
+
   test_sbt:
     group: test
     image: lampepfl/dotty:2018-10-01

build.sbt

@@ -20,6 +20,8 @@ val scalap = Build.scalap
 val dist = Build.dist
 val `dist-bootstrapped` = Build.`dist-bootstrapped`
 
+val sjsSandbox = Build.sjsSandbox
+
 val `sbt-dotty` = Build.`sbt-dotty`
 val `vscode-dotty` = Build.`vscode-dotty`