Manually enqueue compareTo, hashCode, ==, and toString in the Dart-to-Dart backend to avoid tree sh…

sdk/lib/_internal/compiler/implementation/dart_backend/backend.dart

Index: sdk/lib/_internal/compiler/implementation/dart_backend/backend.dart
diff --git a/sdk/lib/_internal/compiler/implementation/dart_backend/backend.dart b/sdk/lib/_internal/compiler/implementation/dart_backend/backend.dart
index 13a215950f4be00ea14a6282a75b3c471b759e6a..6dea90437d71fe470488dd037a57663d1598caef 100644
--- a/sdk/lib/_internal/compiler/implementation/dart_backend/backend.dart
+++ b/sdk/lib/_internal/compiler/implementation/dart_backend/backend.dart
@@ -211,7 +211,18 @@ class DartBackend extends Backend {
       ClassElement classElement = coreLibrary.findLocal(new SourceString(name));
       classElement.ensureResolved(compiler);
     }
+    // Enqueue the methods that the VM might invoke on user objects because
+    // we don't trust the resolution to always get these included.
+    world.registerInvocation(
+        new Selector.call(const SourceString("toString"), null, 0));
+    world.registerInvokedGetter(
+        new Selector.getter(const SourceString("hashCode"), null));
+    world.registerInvocation(
+        new Selector.binaryOperator(const SourceString("==")));
+    world.registerInvocation(
+        new Selector.call(const SourceString("compareTo"), null, 1));
   }
+
   void codegen(CodegenWorkItem work) { }
   void processNativeClasses(Enqueuer world,
                             Iterable<LibraryElement> libraries) { }