Backjump on a source or description mismatch.

sdk/lib/_internal/pub/lib/src/solver/version_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.
 
 library version_solver;
 
 import 'dart:async';
 import 'dart:json' as json;
 
 import '../lock_file.dart';
@@ skipping 144 matching lines @@
 /// Base class for all failures that can occur while trying to resolve versions.
 class SolveFailure implements Exception {
   /// The name of the package whose version could not be solved. Will be `null`
   /// if the failure is not specific to one package.
   final String package;
 
   /// The known dependencies on [package] at the time of the failure. Will be
   /// an empty collection if the failure is not specific to one package.
   final Iterable<Dependency> dependencies;
 
+  /// If this is `true`, then it may be possible to avoid this failure by
+  /// adjusting one of the transitive dependencies of a package in
+  /// [dependencies]. Otherwise, [dependencies] themselves are in conflict and
+  /// only changing one of those may resolve the failure.
+  bool get mayHaveTransitiveCause => true;
+
   SolveFailure(this.package, Iterable<Dependency> dependencies)
       : dependencies = dependencies != null ? dependencies : <Dependency>[];
 
   /// Writes [dependencies] to [buffer] as a bullet list. If [describe] is
   /// passed, it will be called for each dependency and the result will be
   /// written next to the dependency.
   void writeDependencies(StringBuffer buffer,
       [String describe(PackageDep dep)]) {
     var map = {};
     for (var dep in dependencies) {
@@ skipping 93 matching lines @@
   String get _message => "Incompatible version constraints on '$package'";
 }
 
 /// Exception thrown when two packages with the same name but different sources
 /// are depended upon.
 class SourceMismatchException extends SolveFailure {
 
   SourceMismatchException(String package, Iterable<Dependency> dependencies)
       : super(package, dependencies);
 
+  bool get mayHaveTransitiveCause => false;
+
   String get _message => "Incompatible dependencies on '$package'";
 
   String _describeDependency(PackageDep dep) =>
       "depends on it from source ${dep.source}";
 }
 
 /// Exception thrown when two packages with the same name and source but
 /// different descriptions are depended upon.
 class DescriptionMismatchException extends SolveFailure {
   DescriptionMismatchException(String package,
       Iterable<Dependency> dependencies)
       : super(package, dependencies);
 
+  bool get mayHaveTransitiveCause => false;
+
   String get _message => "Incompatible dependencies on '$package'";
 
   String _describeDependency(PackageDep dep) {
     // TODO(nweiz): Dump descriptions to YAML when that's supported.
     return "depends on it with description ${json.stringify(dep.description)}";
   }
 }