Add a test for operator[] unparse.

lib/compiler/implementation/tree/unparser.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.
 
 class Unparser implements Visitor {
   StringBuffer sb;
 
   Unparser();
 
   String unparse(Node node) {
@@ skipping 68 matching lines @@
 
   visitFunctionDeclaration(FunctionDeclaration node) {
     visit(node.function);
   }
 
   visitFunctionExpression(FunctionExpression node) {
     if (node.returnType !== null) {
       visit(node.returnType);
       sb.add(' ');
     }
-    visit(node.name);
+    // TODO(antonm): that's a workaround as currently FunctionExpression
+    // names are modelled with Send and it emits operator[] as only
+    // operator, without [] which are expected to be emitted with
+    // arguments.
+    Send send = node.name.asSend();
+    if (send !== null) {
+      assert(send is !SendSet);
+      visit(send.receiver);
+      visit(send.selector);
+    } else {
+      visit(node.name);
+    }
     visit(node.parameters);
     visit(node.body);
   }
 
   visitIdentifier(Identifier node) {
     add(node.token.value);
   }
 
   visitIf(If node) {
     add(node.ifToken.value);
@@ skipping 332 matching lines @@
       sb.add(' ');
     }
     visit(node.name);
     if (node.typeParameters !== null) {
       visit(node.typeParameters);
     }
     visit(node.formals);
     add(node.endToken.value);
   }
 }