Remove corelib/corelib_strong and migrate last two remaining tests.

tools/migration/lib/src/io.dart

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
 
 /// True if the file system should be left untouched.
 bool dryRun = false;
@@ skipping 30 matching lines @@
     return;
   }
 
   new File(p.join(testRoot, path)).deleteSync();
 }
 
 /// Returns a list of the paths to all files within [dir], which is
 /// assumed to be relative to the SDK's "tests" directory and having file
 /// [extension].
 Iterable<String> listFiles(String dir, {String extension = ".dart"}) {
-  return new Directory(p.join(testRoot, dir))
-      .listSync(recursive: true)
-      .map((entry) {
-    if (!entry.path.endsWith(extension)) return null;
-
-    return entry.path;
-  }).where((path) => path != null);
+  try {
+    return new Directory(p.join(testRoot, dir))
+        .listSync(recursive: true)
+        .map((entry) {
+      if (!entry.path.endsWith(extension)) return null;
+      return entry.path;
+    }).where((path) => path != null);
+  } catch (FileSystemException) {
+    return [];
+  }
 }