Use backtracking when solving dependency constraints.

utils/pub/log.dart

Index: utils/pub/log.dart
diff --git a/utils/pub/log.dart b/utils/pub/log.dart
index ec3fee9798944e586b359b1a0ab0891f78f8d0e0..f0d6b04713e94ef611d7d985d9e089fbdf739970 100644
--- a/utils/pub/log.dart
+++ b/utils/pub/log.dart
@@ -36,6 +36,9 @@ class Level {
   /// operation, process spawning, or file IO.
   static const IO = const Level._("IO  ");
 
+  /// Incremental output during pub's version constraint solver.
+  static const SOLVER = const Level._("SLVR");
+
   /// Fine-grained and verbose additional information. Can be used to provide
   /// program state context for other logs (such as what pub was doing when an
   /// IO operation occurred) or just more detail for an operation.
@@ -68,6 +71,9 @@ void message(message) => write(Level.MESSAGE, message);
 /// Logs [message] at [Level.IO].
 void io(message) => write(Level.IO, message);
 
+/// Logs [message] at [Level.SOLVER].
+void solver(message) => write(Level.SOLVER, message);
+
 /// Logs [message] at [Level.FINE].
 void fine(message) => write(Level.FINE, message);
 
@@ -168,6 +174,7 @@ void showNormal() {
   _loggers[Level.WARNING] = _logToStderr;
   _loggers[Level.MESSAGE] = _logToStdout;
   _loggers[Level.IO]      = null;
+  _loggers[Level.SOLVER]  = null;
   _loggers[Level.FINE]    = null;
 }
 
@@ -178,6 +185,18 @@ void showIO() {
   _loggers[Level.WARNING] = _logToStderrWithLabel;
   _loggers[Level.MESSAGE] = _logToStdoutWithLabel;
   _loggers[Level.IO]      = _logToStderrWithLabel;
+  _loggers[Level.SOLVER]  = null;
+  _loggers[Level.FINE]    = null;
+}
+
+/// Sets the verbosity to "solver", which shows errors, warnings, messages, and
+/// solver logs.
+void showSolver() {
+  _loggers[Level.ERROR]   = _logToStderr;
+  _loggers[Level.WARNING] = _logToStderr;
+  _loggers[Level.MESSAGE] = _logToStdout;
+  _loggers[Level.IO]      = null;
+  _loggers[Level.SOLVER]  = _logToStdout;
   _loggers[Level.FINE]    = null;
 }
 
@@ -187,6 +206,7 @@ void showAll() {
   _loggers[Level.WARNING] = _logToStderrWithLabel;
   _loggers[Level.MESSAGE] = _logToStdoutWithLabel;
   _loggers[Level.IO]      = _logToStderrWithLabel;
+  _loggers[Level.SOLVER]  = _logToStderrWithLabel;
   _loggers[Level.FINE]    = _logToStderrWithLabel;
 }