Remember previously failed packages when backtracking.

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

Index: sdk/lib/_internal/pub/lib/src/solver/version_queue.dart
diff --git a/sdk/lib/_internal/pub/lib/src/solver/version_queue.dart b/sdk/lib/_internal/pub/lib/src/solver/version_queue.dart
index 021b60e246bfaf2f94363d64b46352df01aa89c4..8af2a305b88e0dbeffacecc7e2bbb8b6157b7b72 100644
--- a/sdk/lib/_internal/pub/lib/src/solver/version_queue.dart
+++ b/sdk/lib/_internal/pub/lib/src/solver/version_queue.dart
@@ -40,6 +40,15 @@ class VersionQueue {
     return _allowed.first;
   }
 
+  /// Whether the currently selected version has been responsible for a solve
+  /// failure, or depends on a package that has.
+  ///
+  /// The solver uses this to determine which packages to backtrack to after a
+  /// failure occurs. Any selected package that did *not* cause the failure can
+  /// by skipped by the backtracker.

[nweiz, 2014/05/12 20:18:55]

"by skipped" -> "be skipped"

[Bob Nystrom, 2014/05/12 22:07:55]

Done.

+  bool get hasFailed => _hasFailed;
+  bool _hasFailed = false;
+
   /// Creates a new [VersionQueue] queue for starting with the optional
   /// [locked] package followed by the results of calling [allowedGenerator].
   ///
@@ -66,6 +75,10 @@ class VersionQueue {
   /// Returns `true` if it moved to a new version (which can be accessed from
   /// [current]. Returns `false` if there are no more versions.
   Future<bool> advance() {
+    // Any failure was the fault of the previous version, not necessarily the
+    // new one.
+    _hasFailed = false;
+
     // If we have a locked version, consume it first.
     if (_locked != null) {
       // Advancing past the locked version, so need to load the others now
@@ -81,6 +94,12 @@ class VersionQueue {
     return new Future.value(_allowed.isNotEmpty);
   }
 
+  /// Marks the selected version as being directly or indirectly responsible
+  /// for a solve failure.
+  void fail() {
+    _hasFailed = true;
+  }
+
   /// Determines the list of allowed versions matching its constraint and places
   /// them in [_allowed].
   Future _calculateAllowed() {