Show the dependender(s) for missing packages.

sdk/lib/_internal/pub/test/implicit_barback_dependency_test.dart

 // Copyright (c) 2013, 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 'descriptor.dart' as d;
 import 'test_pub.dart';
 import '../lib/src/barback.dart' as barback;
 import '../lib/src/version.dart';
 
 main() {
@@ skipping 96 matching lines @@
 
   integration("includes pub in the error if a solve failed because there "
       "is no version available", () {
     servePackages([
       packageMap("barback", previous)
     ]);
 
     d.appDir({"barback": "any"}).create();
 
     pubGet(error: """
-Package 'barback' has no versions that match >=$current <$max derived from:
-- 'myapp' depends on version any
+Package barback has no versions that match >=$current <$max derived from:
+- myapp depends on version any
 - pub itself depends on version >=$current <$max""");
   });
 
   integration("includes pub in the error if a solve failed because there "
       "is a disjoint constraint", () {
     servePackages([
       packageMap("barback", current)
     ]);
 
     d.appDir({"barback": previous}).create();
 
     pubGet(error: """
-Incompatible version constraints on 'barback':
-- 'myapp' depends on version $previous
+Incompatible version constraints on barback:
+- myapp depends on version $previous
 - pub itself depends on version >=$current <$max""");
   });
 }