First step towards first-class Scala.js unions

Scala.js uses a magic trait to represent unions in Scala 2, we would
like to unpickle these pseudo-unions as real OrTypes, but this requires
erasing them like Scala 2 does. Now that we have a Scala 2 erasure mode
this is actually possible and this commit adds the necessary erasure
rule (the unpickling side and any change in the Scala.js phases is left
as future work).

Note that we really need to check both that we're in Scala 2 mode and
that -scalajs is enabled, because a regular Scala 2 method type as seen
from a Scala 3 prefix could contain unions after substitutions.

Author: smarter

compiler/src/dotty/tools/dotc/core/TypeErasure.scala

@@ -6,6 +6,7 @@ import Symbols._, Types._, Contexts._, Flags._, Names._, StdNames._, Phases._
 import Flags.JavaDefined
 import Uniques.unique
 import TypeOps.makePackageObjPrefixExplicit
+import backend.sjs.JSDefinitions
 import transform.ExplicitOuter._
 import transform.ValueClasses._
 import transform.TypeUtils._
@@ -506,7 +507,10 @@ class TypeErasure(sourceLanguage: SourceLanguage, semiEraseVCs: Boolean, isConst
     case AndType(tp1, tp2) =>
       erasedGlb(this(tp1), this(tp2), sourceLanguage.isJava)
     case OrType(tp1, tp2) =>
-      TypeComparer.orType(this(tp1), this(tp2), isErased = true)
+      if sourceLanguage.isScala2 && ctx.settings.scalajs.value then
+        JSDefinitions.jsdefn.PseudoUnionType
+      else
+        TypeComparer.orType(this(tp1), this(tp2), isErased = true)
     case tp: MethodType =>
       def paramErasure(tpToErase: Type) =
         erasureFn(sourceLanguage, semiEraseVCs, isConstructor, wildcardOK)(tpToErase)