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

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 f77d22cdb197d3f6b83b7506cdb401da358d7759..420048063915bd3ed6910a1c74c8e7a19015d024 100644
--- a/sdk/lib/_internal/pub/test/version_solver_test.dart
+++ b/sdk/lib/_internal/pub/test/version_solver_test.dart
@@ -402,6 +402,16 @@ unsolvable() {
       '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() {