Fix compiling with the 2.13 collections

- Always rely on -Ynew-collections to change our behavior, attempting to
automatically detect which version we're on leads to cycles.
- In the new collections, the scala.Serializable class has been replaced
by a type alias of java.io.Serializable

Author: smarter

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

@@ -397,8 +397,7 @@ class Definitions {
     def newArrayMethod(implicit ctx: Context): TermSymbol = DottyArraysModule.requiredMethod("newArray")
 
   // TODO: Remove once we drop support for 2.12 standard library
-  lazy val isNewCollections: Boolean = ctx.settings.YnewCollections.value ||
-    ctx.base.staticRef("scala.collection.IterableOnce".toTypeName).exists
+  lazy val isNewCollections: Boolean = ctx.settings.YnewCollections.value
 
   def getWrapVarargsArrayModule: Symbol = if (isNewCollections) ScalaRuntimeModule else ScalaPredefModule
 
@@ -585,7 +584,11 @@ class Definitions {
 
   lazy val ThrowableType: TypeRef          = ctx.requiredClassRef("java.lang.Throwable")
   def ThrowableClass(implicit ctx: Context): ClassSymbol = ThrowableType.symbol.asClass
-  lazy val SerializableType: TypeRef       = ctx.requiredClassRef("scala.Serializable")
+  lazy val SerializableType: TypeRef       =
+    if (isNewCollections)
+      JavaSerializableClass.typeRef
+    else
+      ctx.requiredClassRef("scala.Serializable")
   def SerializableClass(implicit ctx: Context): ClassSymbol = SerializableType.symbol.asClass
   lazy val StringBuilderType: TypeRef      = ctx.requiredClassRef("scala.collection.mutable.StringBuilder")
   def StringBuilderClass(implicit ctx: Context): ClassSymbol = StringBuilderType.symbol.asClass