use const Link so that empty Link is always identical

BUG=
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Review URL: https://codereview.chromium.org/2033873002 .

Author: harryterkelsen

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.