add crash diagnostics in ExtractDependencies

Author: odersky

compiler/src/dotty/tools/dotc/sbt/ExtractDependencies.scala

@@ -335,7 +335,7 @@ private class ExtractDependenciesCollector extends tpd.TreeTraverser { thisTreeT
   /** Traverse the tree of a source file and record the dependencies and used names which
    *  can be retrieved using `dependencies` and`usedNames`.
    */
-  override def traverse(tree: Tree)(implicit ctx: Context): Unit = {
+  override def traverse(tree: Tree)(implicit ctx: Context): Unit = try {
     tree match {
       case Match(selector, _) =>
         addPatMatDependency(selector.tpe)
@@ -384,6 +384,10 @@ private class ExtractDependenciesCollector extends tpd.TreeTraverser { thisTreeT
       case _ =>
         traverseChildren(tree)
     }
+  } catch {
+    case ex: AssertionError =>
+      println(i"asserted failed while traversing $tree")
+      throw ex
   }
 
   /** Traverse a used type and record all the dependencies we need to keep track