Rewrite pub's version solver.

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:pub_semver/pub_semver.dart';
 import 'package:unittest/unittest.dart';
@@ skipping 121 matching lines @@
       'bar': '1.0.0'
     },
     'bar 1.0.0': {
       'foo': '1.0.0'
     }
   }, result: {
     'myapp from root': '1.0.0',
     'foo': '1.0.0',
     'bar': '1.0.0'
   });
+
+  testResolve('removed dependency', {
+    'myapp 1.0.0': {
+      'foo': '1.0.0',
+      'bar': 'any'
+    },
+    'foo 1.0.0': {},
+    'foo 2.0.0': {},
+    'bar 1.0.0': {},
+    'bar 2.0.0': {
+      'baz': '1.0.0'
+    },
+    'baz 1.0.0': {
+      'foo': '2.0.0'
+    }
+  }, result: {
+    'myapp from root': '1.0.0',
+    'foo': '1.0.0',
+    'bar': '1.0.0'
+  }, maxTries: 2);
 }
 
 withLockFile() {
   testResolve('with compatible locked dependency', {
     'myapp 0.0.0': {
       'foo': 'any'
     },
     'foo 1.0.0': { 'bar': '1.0.0' },
     'foo 1.0.1': { 'bar': '1.0.1' },
     'foo 1.0.2': { 'bar': '1.0.2' },
@@ skipping 280 matching lines @@
       "collection": ">=0.9.1 <1.0.0"
     },
     "angular 0.9.10": {
       "di": ">=0.0.32 <0.1.0",
       "collection": ">=0.9.1 <1.0.0"
     },
     "collection 0.9.0": {},
     "collection 0.9.1": {},
     "di 0.0.37": {"analyzer": ">=0.13.0 <0.14.0"},
     "di 0.0.36": {"analyzer": ">=0.13.0 <0.14.0"}
-  }, error: noVersion(['myapp', 'angular', 'collection']), maxTries: 9);
+  }, error: noVersion(['analyzer', 'di']), maxTries: 9);
 }
 
 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 1060 matching lines @@
   }
 
   var source = "mock1";
   if (match[7] != null) {
     source = match[7];
     if (source == "root") source = null;
   }
 
   return new PackageId(name, source, parsedVersion, description);
 }