Address Dmitri's comments on #3600

Author: CodaFi

stdlib/public/core/Sequence.swift

@@ -721,6 +721,14 @@ internal class _PrefixSequence<Base : IteratorProtocol>
         maxLength: Swift.min(maxLength, self._maxLength),
         taken: _taken))
   }
+  
+  internal func drop(
+    while predicate: (Base.Element) throws -> Bool
+  ) rethrows -> AnySequence<Base.Element> {
+    return try AnySequence(
+      _DropWhileSequence(
+        iterator: _iterator, nextElement: nil, predicate: predicate))
+  }
 }
 
 /// A sequence that lazily consumes and drops `n` elements from an underlying
@@ -1253,8 +1261,8 @@ extension Sequence where
   ///
   /// - Parameter predicate: A closure that takes an element of the
   ///   sequence as its argument and returns `true` if the element should
-  ///		be included or `false` if it should be excluded. Once the predicate
-  ///		returns `false` it will not be called again.
+  ///   be included or `false` if it should be excluded. Once the predicate
+  ///   returns `false` it will not be called again.
   ///
   /// - Complexity: O(*n*), where *n* is the length of the collection.
   public func prefix(

validation-test/stdlib/Lazy.swift.gyb

@@ -1295,16 +1295,6 @@ let prefixDropWhileTests: [(data: [Int], value: Int, pivot: Int)] = [
   ([0, 10, 20, 30, 40],     40,     4),
   ([0, 10, 20, 30, 40],     99,     5) ]
 
-tests.test("LazyPrefixWhileIterator") {
-  let base = (1...10).lazy.prefix(while: { $0 < 4 })
-
-  var iter1 = base.makeIterator()
-  expectEqual(1, iter1.next())
-  expectEqual(2, iter1.next())
-  expectEqual(3, iter1.next())
-  expectEqual(nil, iter1.next())
-}
-
 % for Kind in 'Sequence', 'Forward', 'Bidirectional':
 %   Self = 'Sequence' if Kind == 'Sequence' else collectionForTraversal(Kind)
 %   checkKind = 'ForwardCollection' if Kind == 'Forward' else Self
@@ -1365,16 +1355,6 @@ tests.test("LazyPrefixWhileBidirectionalCollection/AssociatedTypes") {
 
 //===--- LazyDropWhile ----------------------------------------------------===//
 
-tests.test("LazyDropWhileIterator") {
-  let base = (1...10).lazy.drop(while: { $0 < 4 })
-
-  var iter1 = base.makeIterator()
-  expectEqual(4, iter1.next())
-  expectEqual(5, iter1.next())
-  expectEqual(6, iter1.next())
-  expectEqual(7, iter1.next())
-}
-
 % for Kind in 'Sequence', 'Forward', 'Bidirectional':
 %   Self = 'Sequence' if Kind == 'Sequence' else collectionForTraversal(Kind)
 %   checkKind = 'ForwardCollection' if Kind == 'Forward' else Self