Support runtime check of function types.

sdk/lib/_internal/compiler/implementation/util/link.dart

Index: sdk/lib/_internal/compiler/implementation/util/link.dart
diff --git a/sdk/lib/_internal/compiler/implementation/util/link.dart b/sdk/lib/_internal/compiler/implementation/util/link.dart
index d3f93ca2191f4706f77063c7e69e1a5fe169afa4..53649d327ce0da251e4842a429877079ef4693a4 100644
--- a/sdk/lib/_internal/compiler/implementation/util/link.dart
+++ b/sdk/lib/_internal/compiler/implementation/util/link.dart
@@ -73,7 +73,12 @@ class Link<T> extends Iterable<T> {
 abstract class LinkBuilder<T> {
   factory LinkBuilder() = LinkBuilderImplementation;
 
-  Link<T> toLink();
+  /**
+   * Prepends all elements added to the builder to [tail]. The resulting list is
+   * returned and the builder is cleared.
+   */
+  Link<T> toLink([Link<T> tail = const Link()]);
+
   void addLast(T t);
 
   final int length;