Sped up hashCode by removing megamorphic call to _identityHashCode.

runtime/lib/bigint.dart

Index: runtime/lib/bigint.dart
diff --git a/runtime/lib/bigint.dart b/runtime/lib/bigint.dart
index c21b04a0f1eba31f25d858c4d41db352d1463f5e..0c562d7b730e9b3e9e4649a9636aa8d1252454cf 100644
--- a/runtime/lib/bigint.dart
+++ b/runtime/lib/bigint.dart
@@ -1206,9 +1206,9 @@ class _Bigint extends _IntegerImplementation implements int {
     return r_used;
   }
 
-  int get _identityHashCode {
-    return this;
-  }
+  int get hashCode => this;
+  int get _identityHashCode => this;
+
   int operator ~() {
     return _not()._toValidInt();
   }