Support optional function expression parameters in dart2js.

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

 // Copyright (c) 2011, 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 dart2js;
 
 class TreeValidatorTask extends CompilerTask {
   TreeValidatorTask(Compiler compiler) : super(compiler);
 
   void validate(Node tree) {
@@ skipping 24 matching lines @@
   }
 
   visitNode(Node node) {}
 
   visitSendSet(SendSet node) {
     final selector = node.selector;
     final name = node.assignmentOperator.source.stringValue;
     final arguments = node.arguments;
 
     expect(node, arguments != null);
-    expect(node, selector is Identifier, 'selector is not assignable');
+    expect(node,
+           selector is Identifier || selector is FunctionExpression,
+           'selector is not assignable');
     if (identical(name, '++') || identical(name, '--')) {
       expect(node, node.assignmentOperator is Operator);
       if (node.isIndex) {
         expect(node.arguments.tail.head, node.arguments.tail.isEmpty);
       } else {
         expect(node.arguments.head, node.arguments.isEmpty);
       }
     } else {
       expect(node, !node.arguments.isEmpty);
     }
@@ skipping 13 matching lines @@
   final String message;
   InvalidNodeError(this.node, [this.message]);
 
   toString() {
     String nodeString = node.toDebugString();
     String result = 'invalid node: $nodeString';
     if (message != null) result = '$result ($message)';
     return result;
   }
 }