Stop using the Hashable interface.

tests/compiler/dart2js_foreign/native_checked_arguments1_test.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // Test that type checks occur on native methods.
 
 @native("*A")
-class A  {
+class A {
   @native int foo(int x);
   @native int cmp(A other);
 }
 
 @native("*B")
-class B  {
+class B {
   @native String foo(String x);
   @native int cmp(B other);
 }
 
 @native A makeA() { return new A(); }
 @native B makeB() { return new B(); }
 
 @native("""
 function A() {}
 A.prototype.foo = function (x) { return x + 1; };
@@ skipping 109 matching lines @@
 
 main() {
   setup();
 
   if (isCheckedMode()) {
     checkedModeTest();
   } else {
     uncheckedModeTest();
   }
 }