Fix bot redness.

sdk/lib/_internal/compiler/implementation/dart_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 dart_backend;
 
 // TODO(ahe): This class is simply wrong.  This backend should use
 // elements when it can, not AST nodes.  Perhaps a [Map<Element,
 // TreeElements>] is what is needed.
 class ElementAst {
@@ skipping 222 matching lines @@
         // from dynamic invocation (alas!).  So we'd better err on preserving
         // those names.
         fixedMemberNames.add(element.name.slowToString());
       });
     }
     // As of now names of named optionals are not renamed. Therefore add all
     // field names used as named optionals into [fixedMemberNames].
     for (final element in resolvedElements.keys) {
       if (!element.isConstructor()) continue;
       Link<Element> optionalParameters =
-          element.functionSignature.optionalParameters;
+          element.computeSignature(compiler).optionalParameters;
       for (final optional in optionalParameters) {
         if (optional.kind != ElementKind.FIELD_PARAMETER) continue;
         fixedMemberNames.add(optional.name.slowToString());
       }
     }
     // The VM will automatically invoke the call method of objects
     // that are invoked as functions. Make sure to not rename that.
     fixedMemberNames.add('call');
     // TODO(antonm): TypeError.srcType and TypeError.dstType are defined in
     // runtime/lib/error.dart. Overall, all DartVM specific libs should be
@@ skipping 292 matching lines @@
 }
 
 compareElements(e0, e1) {
   int result = compareBy((e) => e.getLibrary().canonicalUri.toString())(e0, e1);
   if (result != 0) return result;
   return compareBy((e) => e.position().charOffset)(e0, e1);
 }
 
 List<Element> sortElements(Iterable<Element> elements) =>
     sorted(elements, compareElements);