Rewrite pub's version solver.

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

Index: sdk/lib/_internal/pub/test/version_solver_test.dart
diff --git a/sdk/lib/_internal/pub/test/version_solver_test.dart b/sdk/lib/_internal/pub/test/version_solver_test.dart
index cee9a96a9a80e6638bb6c38f5c6a7fc170995386..a22ca43c1fd70e7212ed43672f0989c29a256675 100644
--- a/sdk/lib/_internal/pub/test/version_solver_test.dart
+++ b/sdk/lib/_internal/pub/test/version_solver_test.dart
@@ -139,6 +139,26 @@ void basicGraph() {
     '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() {
@@ -439,7 +459,7 @@ unsolvable() {
     "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() {