Fix missing core libs test

Rearrange initialization sequence so that we fail with a missing core libs exception
before any other symbols are forced.

Author: odersky

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

@@ -1104,13 +1104,15 @@ class Definitions {
   def init()(implicit ctx: Context) = {
     this.ctx = ctx
     if (!_isInitialized) {
-      // force initialization of every symbol that is synthesized or hijacked by the compiler
-      val forced = syntheticCoreClasses ++ syntheticCoreMethods ++ ScalaValueClasses()
-
       // Enter all symbols from the scalaShadowing package in the scala package
       for (m <- ScalaShadowingPackageClass.info.decls)
         ScalaPackageClass.enter(m)
 
+      // force initialization of every symbol that is synthesized or hijacked by the compiler
+      // Placed here so that an abort with a MissingCoreLibraryException because scalaShadowing
+      // is not found comes before any additional errors are reported.
+      val forced = syntheticCoreClasses ++ syntheticCoreMethods ++ ScalaValueClasses()
+
       _isInitialized = true
     }
   }