Show the dependender(s) for missing packages.

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS d.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 d.file.
 
 library pub_tests;
 
 import 'dart:convert';
 
 import 'descriptor.dart' as d;
 import 'test_pub.dart';
 
 main() {
   initConfig();
 
   forBothPubGetAndUpgrade((command) {
     integration('fails gracefully on a dependency from an unknown source', () {
       d.appDir({"foo": {"bad": "foo"}}).create();
 
       pubCommand(command, error:
-          "Package 'myapp' depends on 'foo' from unknown source 'bad'.");
+          'Package myapp depends on foo from unknown source "bad".');
     });
 
     integration('fails gracefully on transitive dependency from an unknown '
                 'source', () {
       d.dir('foo', [
         d.libDir('foo', 'foo 0.0.1'),
         d.libPubspec('foo', '0.0.1', deps: {"bar": {"bad": "bar"}})
       ]).create();
 
       d.appDir({"foo": {"path": "../foo"}}).create();
 
       pubCommand(command, error:
-          "Package 'foo' depends on 'bar' from unknown source 'bad'.");
+          'Package foo depends on bar from unknown source "bad".');
     });
 
     integration('ignores unknown source in lockfile', () {
       d.dir('foo', [
         d.libDir('foo'),
         d.libPubspec('foo', '0.0.1')
       ]).create();
 
       // Depend on "foo" from a valid source.
       d.dir(appPath, [
@@ skipping 21 matching lines @@
 
       // Should upgrade to the new one.
       d.dir(packagesPath, [
         d.dir("foo", [
           d.file("foo.dart", 'main() => "foo";')
         ])
       ]).validate();
     });
   });
 }