use const Link so that empty Link is always identical

pkg/compiler/lib/src/util/link_implementation.dart

Index: pkg/compiler/lib/src/util/link_implementation.dart
diff --git a/pkg/compiler/lib/src/util/link_implementation.dart b/pkg/compiler/lib/src/util/link_implementation.dart
index ac06c17c2be12ce6773a884955da7a696eb1e5c8..186bfffd2106a8d14e138dfab87269315b3d64a6 100644
--- a/pkg/compiler/lib/src/util/link_implementation.dart
+++ b/pkg/compiler/lib/src/util/link_implementation.dart
@@ -61,7 +61,7 @@ class LinkEntry<T> extends Link<T> {
   Link<T> tail;
 
   LinkEntry(T this.head, [Link<T> tail])
-      : this.tail = ((tail == null) ? new Link<T>() : tail);
+      : this.tail = ((tail == null) ? const Link() : tail);
 
   Link<T> prepend(T element) {
     // TODO(ahe): Use new Link<T>, but this cost 8% performance on VM.