Support overrides in the version solver.

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

Index: sdk/lib/_internal/pub/lib/src/solver/version_solver.dart
diff --git a/sdk/lib/_internal/pub/lib/src/solver/version_solver.dart b/sdk/lib/_internal/pub/lib/src/solver/version_solver.dart
index 83c33256d949159f71364f80ac9a899ab8369325..30bd2d09ecd6f7b38373336b29855195b5c3f4d6 100644
--- a/sdk/lib/_internal/pub/lib/src/solver/version_solver.dart
+++ b/sdk/lib/_internal/pub/lib/src/solver/version_solver.dart
@@ -25,12 +25,14 @@ import 'backtracking_solver.dart';
 /// packages will be used. This is for forcing an upgrade to one or more
 /// packages.
 Future<SolveResult> resolveVersions(SourceRegistry sources, Package root,
-    {LockFile lockFile, List<String> useLatest}) {
+    {LockFile lockFile, List<PackageRef> overrides, List<String> useLatest}) {
   if (lockFile == null) lockFile = new LockFile.empty();
+  if (overrides == null) overrides = [];
   if (useLatest == null) useLatest = [];
 
   return log.progress('Resolving dependencies', () {
-    return new BacktrackingSolver(sources, root, lockFile, useLatest).solve();
+    return new BacktrackingSolver(sources, root, lockFile, overrides,
+        useLatest).solve();
   });
 }