Mark ContextRoot.pathContext constructor argument as required.

This should be a non-breaking change since this is an internal
analyzer API.  All clients that were making use of this internal API
have been updated to pass in the required argument.

Change-Id: Iec81454f6382c8de9ff6340d372338e678a3c73a
Reviewed-on: https://dart-review.googlesource.com/57920
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>
diff --git a/pkg/analyzer/lib/src/context/context_root.dart b/pkg/analyzer/lib/src/context/context_root.dart
index ebb208b7..744f298 100644
--- a/pkg/analyzer/lib/src/context/context_root.dart
+++ b/pkg/analyzer/lib/src/context/context_root.dart
@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:analyzer/src/generated/utilities_general.dart';
+import 'package:meta/meta.dart';
 import 'package:path/path.dart' as path;
 
 /**
@@ -38,7 +39,7 @@
   /**
    * Initialize a newly created context root.
    */
-  ContextRoot(this.root, this.exclude, {path.Context pathContext})
+  ContextRoot(this.root, this.exclude, {@required path.Context pathContext})
       : pathContext = pathContext ?? path.context;
 
   @override