Remove IEquatable.

Runtime/foundation/key.cs

 using System.Collections.Generic;
 
 namespace Unity.UIWidgets.foundation {
-    public abstract class Key : IEquatable<Key> {
+    public abstract class Key {
-        }
-
-        public virtual bool Equals(Key other) {
-            return base.Equals(other);
         }
 
         public static bool operator ==(Key left, Key right) {
         }
 
         public readonly T value;
-
-        public override bool Equals(Key other) {
-            if (ReferenceEquals(null, other)) {
-                return false;
-            }
-
-            if (ReferenceEquals(this, other)) {
-                return true;
-            }
-
-            if (other.GetType() != typeof(ValueKey<T>)) {
-                return false;
-            }
-
-            ValueKey<T> valueKey = (ValueKey<T>) other;
-
-            return this.Equals(valueKey);
-        }
 
         public bool Equals(ValueKey<T> other) {
             if (ReferenceEquals(null, other)) {