Full bootstrap: Use Dotty as the reference compiler

The non-bootstrapped compiler is now compiled by Dotty 0.14.0-RC1
instead of Scala 2.12.8. The bootstrapped compiler is compiled using the
non-bootstrapped compiler as before.

For now, we still support compilation with Scala 2.12, this is achieved
by having separate `src-2.x` and `src-3.x` directory for the library.

To use Scala 2.12 as the reference compiler, run:

++2.12.8

in sbt, or change `referenceVersion` to "2.12.8" in Build.scala.

Author: smarter

.drone.yml

@@ -52,6 +52,13 @@ pipeline:
       # sbt scripted tests are slow and only run on nightly or deployment
       event: [ tag, deployment ]
 
+  test_scala212:
+    group: test
+    image: lampepfl/dotty:2019-02-06
+    commands:
+      - cp -R . /tmp/5/ && cd /tmp/5/
+      - ./project/scripts/sbt ";++2.12.8 ;compile ;test"
+
   # DOCUMENTATION:
   documentation:
     image: lampepfl/dotty:2019-02-06

compiler/test/dotty/tools/TestSources.scala

@@ -65,7 +65,6 @@ object TestSources {
         .map(_.toString)
         .toList
 
-      assert(sources.nonEmpty)
       sources
     }
     finally files.close()

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -232,7 +232,7 @@ class CompilationTests extends ParallelTesting {
       Array("-Ycheck-reentrant", "-Yemit-tasty-in-class")
     )
 
-    val libraryDirs = List(Paths.get("library/src"), Paths.get("library/src-bootstrapped"))
+    val libraryDirs = List(Paths.get("library/src"), Paths.get("library/src-3.x"), Paths.get("library/src-bootstrapped"))
     val librarySources = libraryDirs.flatMap(sources(_))
 
     val lib =