Support optional function expression parameters in dart2js.

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

Index: sdk/lib/_internal/compiler/implementation/tree_validator.dart
diff --git a/sdk/lib/_internal/compiler/implementation/tree_validator.dart b/sdk/lib/_internal/compiler/implementation/tree_validator.dart
index 6a3b93f3776e779cde22c1f0b7ea12ec6c35abba..ce3809eb8559c80d0b90c936e7f7fe27a16a649a 100644
--- a/sdk/lib/_internal/compiler/implementation/tree_validator.dart
+++ b/sdk/lib/_internal/compiler/implementation/tree_validator.dart
@@ -42,7 +42,9 @@ class ValidatorVisitor extends Visitor {
     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) {