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 200 matching lines @@
 
   /// A message describing the specific kind of solve failure.
   String get _message;
 
   /// Describes a dependencie's reference in the output message. Override this
   /// to highlight which aspect of [dep] led to the failure.
   String _describeDependency(PackageDep dep) =>
       "depends on version ${dep.constraint}";
 }
 
-/// Exception thrown when the [VersionSolver] fails to find a solution after a
-/// certain number of iterations.
-class CouldNotSolveException extends SolveFailure {
-  CouldNotSolveException([String message])
-      : super(null, null),
-        _message = (message != null) ? message :
-            "Could not find a solution that met all constraints.";
+/// Exception thrown when the current SDK's version does not match a package's
+/// constraint on it.
+class BadSdkVersionException extends SolveFailure {
+  BadSdkVersionException(String package, String message)
+      : super(package, null),
+        _message = message;
 
   /// A message describing the specific kind of solve failure.
   final String _message;
 }
 
 /// Exception thrown when the [VersionConstraint] used to match a package is
 /// valid (i.e. non-empty), but there are no available versions of the package
 /// that fit that constraint.
 class NoVersionException extends SolveFailure {
   final VersionConstraint constraint;
@@ skipping 50 matching lines @@
       Iterable<Dependency> dependencies)
       : super(package, dependencies);
 
   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)}";
   }
 }