Make pub's binstubs resilient to changes in snapshot format.

sdk/lib/_internal/pub_generated/lib/src/io.dart

Index: sdk/lib/_internal/pub_generated/lib/src/io.dart
diff --git a/sdk/lib/_internal/pub_generated/lib/src/io.dart b/sdk/lib/_internal/pub_generated/lib/src/io.dart
index 8595415fb459d34a6a00a09ace02be23a2b15b97..16682ddf0c97269962928b29138f1cb514267887 100644
--- a/sdk/lib/_internal/pub_generated/lib/src/io.dart
+++ b/sdk/lib/_internal/pub_generated/lib/src/io.dart
@@ -391,6 +391,17 @@ void deleteEntry(String path) {
   });
 }
 
+/// Attempts to delete whatever's at [path], but doesn't throw an exception if
+/// the deletion fails.
+void tryDeleteEntry(String path) {
+  try {
+    deleteEntry(path);
+  } catch (error, stackTrace) {
+    log.fine(
+        "Failed to delete $entry: $error\n" "${new Chain.forTrace(stackTrace)}");
+  }
+}
+
 /// "Cleans" [dir].
 ///
 /// If that directory already exists, it is deleted. Then a new empty directory