Improve circular dependency overrides.

lib/src/solver/backtracking_solver.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.
 
 /// A back-tracking depth-first solver.
 ///
 /// Attempts to find the best solution for a root package's transitive
 /// dependency graph, where a "solution" is a set of concrete package versions.
 /// A valid solution will select concrete versions for every package reached
 /// from the root package's dependency graph, and each of those packages will
@@ skipping 561 matching lines @@
       deps = deps.map((dep) {
         var override = _overrides[dep.name];
         if (override != null) return override;
 
         // Not overridden.
         return dep;
       }).toSet();
     } else {
       // Ignore any overridden dependencies.
       deps.removeWhere((dep) => _overrides.containsKey(dep.name));
+
+      // If an overridden dependency depends on the root package, ignore that
+      // dependency. This ensures that users can work on the next version of one
+      // side of a circular dependency easily.
+      if (_overrides.containsKey(id.name)) {
+        deps.removeWhere((dep) => dep.name == root.name);
+      }
     }
 
     // Make sure the package doesn't have any bad dependencies.
     for (var dep in deps.toSet()) {
       if (!dep.isRoot && dep.source is UnknownSource) {
         throw new UnknownSourceException(id.name, [new Dependency(id, dep)]);
       }
 
       if (dep.name == 'barback') {
         deps.add(new PackageDep.magic('pub itself'));
@@ skipping 70 matching lines @@
 
       if (!pubspec.flutterSdkConstraint.allows(flutter.version)) {
         throw new BadSdkVersionException(pubspec.name,
             'Package ${pubspec.name} requires Flutter SDK version '
             '${pubspec.flutterSdkConstraint} but the current SDK is '
             '${flutter.version}.');
       }
     }
   }
 }