Warn about overriding operator== but not hashCode.

dart/tests/compiler/dart2js/resolver_test.dart

 // Copyright (c) 2012, 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.
 
 import "package:expect/expect.dart";
 import 'dart:collection';
 
 import "../../../sdk/lib/_internal/compiler/implementation/resolution/resolution.dart";
 import "compiler_helper.dart";
 import "parser_helper.dart";
@@ skipping 56 matching lines @@
   testNewExpression();
   testTopLevelFields();
   testClassHierarchy();
   testInitializers();
   testThis();
   testSuperCalls();
   testTypeVariables();
   testToString();
   testIndexedOperator();
   testIncrementsAndDecrements();
+  testOverrideHashCodeCheck();
 }
 
 testTypeVariables() {
   matchResolvedTypes(visitor, text, name, expectedElements) {
     VariableDefinitions definition = parseStatement(text);
     visitor.visit(definition.type);
     InterfaceType type = visitor.mapping.getType(definition.type);
     Expect.equals(definition.type.typeArguments.slowLength(),
                   length(type.typeArguments));
     int index = 0;
@@ skipping 751 matching lines @@
         var d = new D();
         --d;
       }""";
   final compiler = compileScript(script);
 
   checkMemberResolved(compiler, 'A', operatorName('+', false));
   checkMemberResolved(compiler, 'B', operatorName('+', false));
   checkMemberResolved(compiler, 'C', operatorName('-', false));
   checkMemberResolved(compiler, 'D', operatorName('-', false));
 }
+
+testOverrideHashCodeCheck() {
+  final script = r"""
+      class A {
+        operator==(other) => true;
+      }
+      class B {
+        operator==(other) => true;
+        get hashCode => 0;
+      }
+      main() {
+        new A() == new B();
+      }""";
+  final compiler = compileScript(script);
+  Expect.equals(1, compiler.warnings.length);
+  Expect.equals(MessageKind.OVERRIDE_EQUALS_NOT_HASH_CODE,
+                compiler.warnings[0].message.kind);
+  Expect.equals(0, compiler.errors.length);
+}