Add a test for operator[] unparse.

lib/compiler/implementation/tree/unparser.dart

Index: lib/compiler/implementation/tree/unparser.dart
diff --git a/lib/compiler/implementation/tree/unparser.dart b/lib/compiler/implementation/tree/unparser.dart
index 67df031e46cd78c9fe67c3c20cac0b7aaf25dcf6..b1ca8d66721bec5f2f2e90a71937cfe7b5e949e7 100644
--- a/lib/compiler/implementation/tree/unparser.dart
+++ b/lib/compiler/implementation/tree/unparser.dart
@@ -86,7 +86,18 @@ class Unparser implements Visitor {
       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);
   }