Update stdlib/public/core/Collection.swift

Co-authored-by: Xiaodi Wu <[email protected]>

Author: stephencelis

stdlib/public/core/Collection.swift

@@ -1664,7 +1664,10 @@ extension Collection where SubSequence == Self {
   public mutating func removeFirst(_ k: Int) {
     if k == 0 { return }
     _precondition(k >= 0, "Number of elements to remove should be non-negative")
-    _precondition(index(startIndex, offsetBy: k, limitedBy: endIndex) != nil,
+    guard let idx = index(startIndex, offsetBy: k, limitedBy: endIndex) else {
+      _preconditionFailure("Can't remove more items from a collection than it contains")
+    }
+    self = self[idx..<endIndex]
       "Can't remove more items from a collection than it contains")
     self = self[index(startIndex, offsetBy: k)..<endIndex]
   }