Incremental compiler: support optional arguments.

dart/pkg/compiler/lib/src/js_backend/backend.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of js_backend;
 
 const VERBOSE_OPTIMIZER_HINTS = false;
 
 const bool USE_CPS_IR = const bool.fromEnvironment("USE_CPS_IR");
 
@@ skipping 2166 matching lines @@
     return staticFields;
   }
 
   /// Called when [enqueuer] is empty, but before it is closed.
   bool onQueueEmpty(Enqueuer enqueuer, Iterable<ClassElement> recentClasses) {
     // Add elements referenced only via custom elements.  Return early if any
     // elements are added to avoid counting the elements as due to mirrors.
     customElementsAnalysis.onQueueEmpty(enqueuer);
     if (!enqueuer.queueIsEmpty) return false;
 
+    if (compiler.hasIncrementalSupport) {
+      // Always enable tear-off closures during incremental compilation.
+      Element e = findHelper('closureFromTearOff');
+      if (e != null && !enqueuer.isProcessed(e)) {
+        registerBackendUse(e);
+        enqueuer.addToWorkList(e);
+      }
+    }
+
     if (!enqueuer.isResolutionQueue && preMirrorsMethodCount == 0) {
       preMirrorsMethodCount = generatedCode.length;
     }
 
     if (isTreeShakingDisabled) {
       enqueuer.enqueueReflectiveElements(recentClasses);
     } else if (!targetsUsed.isEmpty && enqueuer.isResolutionQueue) {
       // Add all static elements (not classes) that have been requested for
       // reflection. If there is no mirror-usage these are probably not
       // necessary, but the backend relies on them being resolved.
@@ skipping 300 matching lines @@
   }
 }
 
 /// Records that [constant] is used by the element behind [registry].
 class Dependency {
   final ConstantValue constant;
   final Element annotatedElement;
 
   const Dependency(this.constant, this.annotatedElement);
 }