Simplify ClassOf phase

Make use of Definitions as the repository for
standard symbols in Predef.

Author: odersky

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

@@ -221,6 +221,8 @@ class Definitions {
 
     lazy val Predef_conformsR = ScalaPredefModule.requiredMethodRef("$conforms")
     def Predef_conforms(implicit ctx: Context) = Predef_conformsR.symbol
+    lazy val Predef_classOfR = ScalaPredefModule.requiredMethodRef("classOf")
+    def Predef_classOf(implicit ctx: Context) = Predef_classOfR.symbol
 
   lazy val ScalaRuntimeModuleRef = ctx.requiredModuleRef("scala.runtime.ScalaRunTime")
   def ScalaRuntimeModule(implicit ctx: Context) = ScalaRuntimeModuleRef.symbol

src/dotty/tools/dotc/transform/ClassOf.scala

@@ -21,15 +21,8 @@ class ClassOf extends MiniPhaseTransform {
 
   override def phaseName: String = "classOf"
 
-  private var classOfMethod: TermSymbol = _
-
-  override def prepareForUnit(tree: tpd.Tree)(implicit ctx: Context): TreeTransform = {
-    classOfMethod = defn.ScalaPredefModule.requiredMethod(nme.classOf)
-    this
-  }
-
   override def transformTypeApply(tree: TypeApply)(implicit ctx: Context, info: TransformerInfo): Tree =
-    if (tree.symbol eq classOfMethod) {
+    if (tree.symbol eq defn.Predef_classOf) {
       val targ = tree.args.head.tpe
       clsOf(targ).ensureConforms(tree.tpe).withPos(tree.pos)
     }