Resolve SeqType based on alias defined in scala package object

  - in 2.12: scala.collection.Seq
  - in 2.13: scala.collection.immutable.Seq

Revert when we drop support for 2.12?

Author: allanrenucci

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

@@ -401,7 +401,14 @@ class Definitions {
       List(AnyClass.typeRef), EmptyScope)
   lazy val SingletonType: TypeRef = SingletonClass.typeRef
 
-  lazy val SeqType: TypeRef = ctx.requiredClassRef("scala.collection.Seq")
+  lazy val SeqType: TypeRef = {
+    // We load SeqType from the alias in scala package object
+    //  - in 2.12: scala.collection.Seq
+    //  - in 2.13: scala.collection.immutable.Seq
+    val alias = ctx.base.staticRef("scala.Seq".toTypeName).requiredSymbol(_.isAliasType)
+    alias.info.classSymbol.typeRef
+  }
+
   def SeqClass(implicit ctx: Context) = SeqType.symbol.asClass
     lazy val Seq_applyR = SeqClass.requiredMethodRef(nme.apply)
     def Seq_apply(implicit ctx: Context) = Seq_applyR.symbol

compiler/src/dotty/tools/dotc/typer/Implicits.scala

@@ -71,7 +71,7 @@ object Implicits {
     /** The implicit references */
     def refs: List[ImplicitRef]
 
-    private var SingletonClass: ClassSymbol = null
+    private[this] var SingletonClass: ClassSymbol = null
 
     /** Widen type so that it is neither a singleton type nor a type that inherits from scala.Singleton. */
     private def widenSingleton(tp: Type)(implicit ctx: Context): Type = {