Check that const-map keys don't override equals.

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

Index: sdk/lib/_internal/compiler/implementation/resolution/members.dart
diff --git a/sdk/lib/_internal/compiler/implementation/resolution/members.dart b/sdk/lib/_internal/compiler/implementation/resolution/members.dart
index 69e3b45b9d02d08514b558f3b894906b061ee331..1ea01a85e38bdbc3d007b9cf76a1c7b54b4d5018 100644
--- a/sdk/lib/_internal/compiler/implementation/resolution/members.dart
+++ b/sdk/lib/_internal/compiler/implementation/resolution/members.dart
@@ -3098,11 +3098,32 @@ class ResolverVisitor extends MappingVisitor<Element> {
     return null;
   }
 
+  void checkConstMapKeysDontOverrideEquals(Spannable spannable,
+                                           MapConstant map) {
+    for (Constant key in map.keys.entries) {
+      if (!key.isObject()) continue;
+      ObjectConstant objectConstant = key;
+      DartType keyType = objectConstant.type;
+      ClassElement cls = keyType.element;
+      if (cls == compiler.stringClass) continue;
+      Element equals = cls.lookupMember('==');
+      if (equals.getEnclosingClass() != compiler.objectClass) {
+        compiler.reportError(spannable,
+                             MessageKind.CONST_MAP_KEY_OVERRIDES_EQUALS,
+                             {'type': keyType});
+      }
+    }
+  }
+
   void analyzeConstant(Node node, {bool isConst: true}) {
     addDeferredAction(enclosingElement, () {
       Constant constant = compiler.constantHandler.compileNodeWithDefinitions(
           node, mapping, isConst: isConst);
 
+      if (isConst && constant != null && constant.isMap()) {
+        checkConstMapKeysDontOverrideEquals(node, constant);
+      }
+
       // The type constant that is an argument to JS_INTERCEPTOR_CONSTANT names
       // a class that will be instantiated outside the program by attaching a
       // native class dispatch record referencing the interceptor.