Get rid of Chain.track in 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 cf12722fcb92ab15a12bfacd77ab8300265533e8..8f19a66287efb94ea20a19029038f20a5fb5b6c5 100644
--- a/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
+++ b/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
@@ -559,7 +559,7 @@ class Traverser {
   /// Register [dependency]'s constraints on the package it depends on and
   /// enqueues the package for processing if necessary.
   Future _registerDependency(Dependency dependency) {
-    return syncFuture(() {
+    return new Future.sync(() {
       _validateDependency(dependency);
 
       var dep = dependency.dep;