Warn about overriding operator== but not hashCode.

dart/sdk/lib/_internal/compiler/implementation/warnings.dart

Index: dart/sdk/lib/_internal/compiler/implementation/warnings.dart
diff --git a/dart/sdk/lib/_internal/compiler/implementation/warnings.dart b/dart/sdk/lib/_internal/compiler/implementation/warnings.dart
index 8f9b9cb7981677a4107bd314a32d0c91cc3b1bf0..9368f89b78795300d2c5a32589128409d46e88c7 100644
--- a/dart/sdk/lib/_internal/compiler/implementation/warnings.dart
+++ b/dart/sdk/lib/_internal/compiler/implementation/warnings.dart
@@ -433,13 +433,17 @@ Error: "#{value}" is not a valid Symbol name because is not:
  * a qualified non-private identifier followed by "." and a user-defined operator.''');
 
   static const AMBIGUOUS_REEXPORT = const MessageKind(
-    'Info: "#{element}" is (re)exported by multiple libraries.');
+      'Info: "#{element}" is (re)exported by multiple libraries.');
 
   static const AMBIGUOUS_LOCATION = const MessageKind(
-    'Info: "#{element}" is defined here.');
+      'Info: "#{element}" is defined here.');
 
   static const IMPORTED_HERE = const MessageKind(
-    'Info: "#{element}" is imported here.');
+      'Info: "#{element}" is imported here.');
+
+  static const OVERRIDE_EQUALS_NOT_HASH_CODE = const MessageKind(
+      'Warning: The class "#{class}" overrides "operator==", '
+      'but not "get hashCode".');
 
   static const COMPILER_CRASHED = const MessageKind(
       "Error: The compiler crashed when compiling this element.");