Make DottyPredef compilable with -Yno-imports

DottyPredef needs to be compiled with -Yno-imports because it would
clash otherwise with the DottyPredef in the root context.

Note that ??? has to be written in fully qualified form because of #530.

Author: odersky

src/dotty/DottyPredef.scala

@@ -2,9 +2,10 @@ package dotty
 
 import scala.reflect.ClassTag
 import scala.reflect.runtime.universe.TypeTag
+import scala.Predef.???  // this is currently ineffective, because of #530
 
 object DottyPredef {
   /** implicits for ClassTag and TypeTag. Should be implemented with macros */
-  implicit def classTag[T]: ClassTag[T] = ???
-  implicit def typeTag[T]: TypeTag[T] = ???
+  implicit def classTag[T]: ClassTag[T] = scala.Predef.???
+  implicit def typeTag[T]: TypeTag[T] = scala.Predef.???
 }