stdlib: Use config directives to work without objective-c.

Swift SVN r23211

stdlib/core/Arrays.swift.gyb

@@ -286,7 +286,15 @@ public struct ${Self}<T> : MutableCollectionType, Sliceable {
   }
 
   public
+%if Self == 'Array':
+#if _runtime(_ObjC)
+  typealias _Buffer = _ArrayBuffer<T>
+#else
+  typealias _Buffer = _ContiguousArrayBuffer<T>
+#endif
+%else:
   typealias _Buffer = _${Self.strip('_')}Buffer<T>
+%end
 
   /// Initialization from an existing buffer does not have "array.init"
   /// semantics because the caller may retain an alias to buffer.
@@ -1139,13 +1147,18 @@ public func !=<T: Equatable>(lhs: ${Self}<T>, rhs: ${Self}<T>) -> Bool {
 }
 %end
 
+#if _runtime(_ObjC)
 /// Returns an Array<Base> containing the same elements as a in
 /// O(1). Requires: Base is a base class or base @objc protocol (such
 /// as AnyObject) of Derived.
+/// FIXME: Dynamic casting is currently not possible without the objc runtime:
+/// rdar://problem/18801510
 public func _arrayUpCast<Derived, Base>(a: Array<Derived>) -> Array<Base> {
   return Array(a._buffer.castToBufferOf(Base.self))
 }
+#endif
 
+#if _runtime(_ObjC)
 extension Array {
   /// Try to downcast the source `NSArray` as our native buffer type.
   /// If it succeeds, create a new `Array` around it and return that.
@@ -1181,7 +1194,7 @@ extension Array {
     self = Array(_ArrayBuffer(selectedSource))
   }
 }
-
+#endif
 
 // ${'Local Variables'}:
 // eval: (read-only-mode 1)