Use backtracking when solving dependency constraints.

utils/tests/pub/pub_test.dart

Index: utils/tests/pub/pub_test.dart
diff --git a/utils/tests/pub/pub_test.dart b/utils/tests/pub/pub_test.dart
index 7e8e3493a6fc50676cfb15aa40dba0376d8e6b3d..36071ba35302a136e300aa13aa5884095a3f7070 100644
--- a/utils/tests/pub/pub_test.dart
+++ b/utils/tests/pub/pub_test.dart
@@ -22,9 +22,10 @@ final USAGE_STRING = """
         --[no-]trace      Print debugging information when an error occurs.
         --verbosity       Control output verbosity.
 
-              [all]       All output including internal tracing messages are shown.
-              [io]        IO operations are also shown.
-              [normal]    Errors, warnings, and user messages are shown.
+              [all]       Show all output including internal tracing messages.
+              [io]        Also show IO operations.
+              [normal]    Show errors, warnings, and user messages.
+              [solver]    Show steps during version resolution.
 
     -v, --verbose         Shortcut for "--verbosity=all"