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

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

Index: sdk/lib/_internal/pub/lib/src/io.dart
diff --git a/sdk/lib/_internal/pub/lib/src/io.dart b/sdk/lib/_internal/pub/lib/src/io.dart
index c55c17b67e345308edbe6b4e2e02fed1787267e2..58a52ef00eaf0e412fb618ec23763f836da0273f 100644
--- a/sdk/lib/_internal/pub/lib/src/io.dart
+++ b/sdk/lib/_internal/pub/lib/src/io.dart
@@ -392,6 +392,17 @@ 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