Warn about overriding operator== but not hashCode.

dart/tests/compiler/dart2js/resolver_test.dart

Index: dart/tests/compiler/dart2js/resolver_test.dart
diff --git a/dart/tests/compiler/dart2js/resolver_test.dart b/dart/tests/compiler/dart2js/resolver_test.dart
index dcdcc070919b5386c606a65aee86a8e0d309f712..1902d65337bc1f47e60112c9a5bfa401ddc7c280 100644
--- a/dart/tests/compiler/dart2js/resolver_test.dart
+++ b/dart/tests/compiler/dart2js/resolver_test.dart
@@ -74,6 +74,7 @@ main() {
   testToString();
   testIndexedOperator();
   testIncrementsAndDecrements();
+  testOverrideHashCodeCheck();
 }
 
 testTypeVariables() {
@@ -845,3 +846,22 @@ testIncrementsAndDecrements() {
   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);
+}