Warn about overriding operator== but not hashCode.

dart/sdk/lib/_internal/compiler/implementation/resolution/members.dart

Index: dart/sdk/lib/_internal/compiler/implementation/resolution/members.dart
diff --git a/dart/sdk/lib/_internal/compiler/implementation/resolution/members.dart b/dart/sdk/lib/_internal/compiler/implementation/resolution/members.dart
index 04aa685fade9e6543db7053012bafd03d4c899a6..dca98d4f965f7f245633f07ecf74089d8d649503 100644
--- a/dart/sdk/lib/_internal/compiler/implementation/resolution/members.dart
+++ b/dart/sdk/lib/_internal/compiler/implementation/resolution/members.dart
@@ -778,6 +778,7 @@ class ResolverTask extends CompilerTask {
     } else if (isBinaryOperator(value)) {
       messageKind = MessageKind.BINARY_OPERATOR_BAD_ARITY;
       requiredParameterCount = 1;
+      if (identical(value, '==')) checkOverrideHashCode(member);
     } else if (isTernaryOperator(value)) {
       messageKind = MessageKind.TERNARY_OPERATOR_BAD_ARITY;
       requiredParameterCount = 2;
@@ -788,6 +789,17 @@ class ResolverTask extends CompilerTask {
     checkArity(function, requiredParameterCount, messageKind, isMinus);
   }
 
+  void checkOverrideHashCode(FunctionElement operatorEquals) {
+    if (operatorEquals.isAbstract(compiler)) return;
+    ClassElement cls = operatorEquals.getEnclosingClass();
+    Element hashCodeImplementation =
+        cls.lookupLocalMember(const SourceString('hashCode'));
+    if (hashCodeImplementation != null) return;
+    compiler.reportLint(
+        operatorEquals, MessageKind.OVERRIDE_EQUALS_NOT_HASH_CODE,
+        {'class': cls.name.slowToString()});
+  }
+
   void checkArity(FunctionElement function,
                   int requiredParameterCount, MessageKind messageKind,
                   bool isMinus) {