Make match exhaustive for bootstrapping

Author: milessabin

compiler/test/dotty/tools/CheckTypeTest.scala

@@ -1,7 +1,7 @@
 package dotty.tools
 
 import org.junit.Test
-import org.junit.Assert.{ assertFalse, assertTrue }
+import org.junit.Assert.{ assertFalse, assertTrue, fail }
 
 import dotc.ast.Trees._
 import dotc.core.Decorators._
@@ -35,6 +35,8 @@ class CheckTypeTest extends DottyTest {
         assertTrue  (ls <:< lr)
         assertTrue  (lb <:< la)
         assertFalse (la <:< lb)
+
+      case _ => fail
     }
   }
 }