Don't migrate status entries under a "[ $strong ]" header.

tools/migration/lib/src/migrate_statuses.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 'package:path/path.dart' as p;
 
 import 'editable_status_file.dart';
 import 'fork.dart';
 import 'io.dart';
 import 'log.dart';
@@ skipping 89 matching lines @@
     if (possibleFiles.length > 1) {
       // The condition matches multiple files, so the user is going to have to
       // manually split it up into multiple sections first.
       // TODO(rnystrom): Would be good to automate this, though it requires
       // being able to work with condition expressions directly.
       var statusRelative = p.relative(fromFile, from: testRoot);
       _todoHeaders.putIfAbsent(statusRelative, () => new Set()).add(header);
       return false;
     }
 
+    // The main "_strong.status" files skip lots of tests that are or were not
+    // strong mode clean. We don't want to skip those tests in 2.0 -- we want
+    // to fix them. If we're in that header, do remove the entry from the old
+    // file, but don't add it to the new one.
+    if (header == "[ \$strong ]") return true;
+
     // If the condition places it directly into one file, put it there.
     if (possibleFiles.length == 1) {
       destination = "${toDir}_${possibleFiles.single}.status";
     }
 
     var sections = _files.putIfAbsent(p.join(toDir, destination), () => {});
 
     var entries = sections.putIfAbsent(header, () => []);
     entries.add(entry);
     return true;
@@ skipping 81 matching lines @@
         break;
       }
     }
 
     if (!found) {
       // This section doesn't exist in the status file, so add it.
       editable.append(header, entries[header]);
     }
   }
 }