Don't let the version solver starve the event queue.

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 73dfd5c4ec640d843de82bd5b77f186543d17ed0..ecea2cd00d9650184acf5d44b61701d08fb62bbc 100644
--- a/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
+++ b/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
@@ -251,20 +251,19 @@ class BacktrackingSolver {
   /// If there are no more versions, continues to backtrack to previous
   /// selections, and so on. If there is nothing left to backtrack to,
   /// completes to the last failure that occurred.
-  Future<List<PackageId>> _traverseSolution() => resetStack(() {
-    return new Traverser(this).traverse().catchError((error) {
-      if (error is! SolveFailure) throw error;
-
-      return _backtrack(error).then((canTry) {
-        if (canTry) {
-          _attemptedSolutions++;
-          return _traverseSolution();
-        }
-
-        // All out of solutions, so fail.
-        throw error;
-      });
-    });
+  Future<List<PackageId>> _traverseSolution() => resetStack(() async {
+    // Avoid starving the event queue by waiting for a timer-level event.
+    await new Future(() {});
+
+    try {
+      return await new Traverser(this).traverse();
+    } on SolveFailure catch (error) {
+      // All out of solutions, so fail.
+      if (!(await _backtrack(error))) rethrow;
+
+      _attemptedSolutions++;
+      await _traverseSolution();
+    }
   });
 
   /// Backtracks from the current failed solution and determines the next