Turn on -language:Scala2 -migration when compiling stdlib

needed to turn some errors into warnings.

Author: odersky

test/dotc/tests.scala

@@ -178,7 +178,7 @@ class tests extends CompilerTest {
    .filter(_.nonEmpty)
    .toList
 
-  @Test def compileStdLib = compileList("compileStdLib", stdlibFiles)
+  @Test def compileStdLib = compileList("compileStdLib", stdlibFiles, "-migration" :: scala2mode)
   @Test def dotty = compileDir(dottyDir, ".", "-deep" :: "-Ycheck-reentrant" :: allowDeepSubtypes) // note the -deep argument
 
   @Test def dotc_ast = compileDir(dotcDir, "ast")