Remove deprecated parts of dart:async.

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 85c52c2fc1718f57c2f6a9dffed0265f001572e9..48b84f2104110c32ef01e3439e179f3a1118e317 100644
--- a/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
+++ b/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
@@ -404,12 +404,11 @@ class Traverser {
 
         return _solver.cache.getVersions(dep.toRef()).then((versions) {
           return new Pair<PackageDep, int>(dep, versions.length);
-        }).catchError((error) {
+        }).catchError((error, trace) {
           // If it fails for any reason, just treat that as no versions. This
           // will sort this reference higher so that we can traverse into it
           // and report the error more properly.
-          log.solver("Could not get versions for $dep:\n$error\n\n"
-              "${getAttachedStackTrace(error)}");
+          log.solver("Could not get versions for $dep:\n$error\n\n$trace");
           return new Pair<PackageDep, int>(dep, 0);
         });
       }