Include stderr on git failure.

utils/pub/version_solver.dart

Index: utils/pub/version_solver.dart
diff --git a/utils/pub/version_solver.dart b/utils/pub/version_solver.dart
index 988e44ce90c5eabc8a77bd10a40ca54941741c5e..8af26aab4abc639a7c04d32ecefd65f73bbc425b 100644
--- a/utils/pub/version_solver.dart
+++ b/utils/pub/version_solver.dart
@@ -197,7 +197,7 @@ class VersionSolver {
         return true;
       }
     }
-    
+
     return dependency.dependers.map(getDependency).some((subdependency) =>
         tryUnlockDepender(subdependency, seen));
   }
@@ -328,7 +328,7 @@ class ChangeVersion implements WorkItem {
  * graph once a dependency has changed. Changing the dependency is the
  * responsibility of subclasses.
  */
-class ChangeConstraint implements WorkItem {
+abstract class ChangeConstraint implements WorkItem {
   abstract Future process(VersionSolver solver);
 
   abstract void undo(VersionSolver solver);