     }
     else {
       self.storage = Builtin.castToNativeObject(
-        IndirectArrayBuffer(nativeBuffer: source, isMutable: true))
+        IndirectArrayBuffer(
+          nativeBuffer: source, isMutable: true, needsElementTypeCheck: false))
     }
   }
   

  var buffer: AnyObject?
  var isMutable: Bool
  var isCocoa: Bool
  
  func getNative<T>() -> ContiguousArrayBuffer<T> {
    _sanityCheck(!isCocoa)

  /// Return a SliceBuffer containing the given subRange of values
  /// from this buffer.
  subscript(subRange: Range<Int>) -> SliceBuffer<T> {
    _typeCheck(subRange)
    
    if _fastPath(_isNative) {
      return _native[subRange]
