Don't crash if a failed dependency is in the root package.

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

 // Copyright (c) 2012, 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.
 
 library pub_upgrade_test;
 
 import 'dart:async';
 
 import 'package:unittest/unittest.dart';
 
@@ skipping 384 matching lines @@
     'a 2.0.0': {
       'b': '2.0.0'
     },
     'b 1.0.0': {
       'a': '2.0.0'
     },
     'b 2.0.0': {
       'a': '1.0.0'
     }
   }, error: couldNotSolve, maxTries: 4);
+
+  // This is a regression test for #15550.
+  testResolve('no version that matches while backtracking', {
+    'myapp 0.0.0': {
+      'a': 'any',
+      'b': '>1.0.0'
+    },
+    'a 1.0.0': {},
+    'b 1.0.0': {}
+  }, error: noVersion(['myapp']), maxTries: 1);
 }
 
 badSource() {
   testResolve('fail if the root package has a bad source in dep', {
     'myapp 0.0.0': {
       'foo from bad': 'any'
     },
   }, error: unknownSource('myapp', 'foo', 'bad'));
 
   testResolve('fail if the root package has a bad source in dev dep', {
@@ skipping 970 matching lines @@
   }
 
   var source = "mock1";
   if (match[7] != null) {
     source = match[7];
     if (source == "root") source = null;
   }
 
   return new PackageId(name, source, parsedVersion, description);
 }