Add a bunch of extra logging to Pub.

sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart

Index: sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
diff --git a/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart b/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
index efb6fba9b2231b368cb133ef46e33165fed6d258..97d35dad9e930cd0bc787dbe2762d4de414e7653 100644
--- a/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
+++ b/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
@@ -98,6 +98,8 @@ class BacktrackingSolver {
   Future<SolveResult> solve() {
     var stopwatch = new Stopwatch();
 
+    _logParameters();
+
     return new Future(() {
       stopwatch.start();
 
@@ -284,6 +286,23 @@ class BacktrackingSolver {
     return walkDeps(depender, depender.name);
   }
 
+  /// Logs the initial parameters to the solver.
+  void _logParameters() {
+    var buffer = new StringBuffer();
+    buffer.writeln("Solving dependencies:");
+    for (var package in root.dependencies) {
+      buffer.write("- $package");
+      if (_forceLatest.contains(package.name)) {
+        buffer.write(" (use latest)");
+      } else if (lockFile.packages.containsKey(package.name)) {
+        var version = lockFile.packages[package.name].version;
+        buffer.write(" (locked to $version)");
+      }
+      buffer.writeln();
+    }
+    log.solver(buffer.toString().trim());
+  }
+
   /// Logs [message] in the context of the current selected packages. If
   /// [message] is omitted, just logs a description of leaf-most selection.
   void logSolve([String message]) {