Fix bot redness.

sdk/lib/_internal/compiler/implementation/dart_backend/placeholder_collector.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;
 
 class LocalPlaceholder {
   final String identifier;
   final Set<Node> nodes;
   LocalPlaceholder(this.identifier) : nodes = new Set<Node>();
@@ skipping 363 matching lines @@
       makeConstructorPlaceholder(node.send.selector, constructor, type);
       // TODO(smok): Should this be in visitNamedArgument?
       // Field names can be exposed as names of optional arguments, e.g.
       // class C {
       //   final field;
       //   C([this.field]);
       // }
       // Do not forget to rename them as well.
       FunctionElement constructorFunction = constructor;
       Link<Element> optionalParameters =
-          constructorFunction.functionSignature.optionalParameters;
+          constructorFunction.computeSignature(compiler).optionalParameters;
       for (final argument in send.argumentsNode) {
         NamedArgument named = argument.asNamedArgument();
         if (named == null) continue;
         Identifier name = named.name;
         String nameAsString = name.source.slowToString();
         for (final parameter in optionalParameters) {
           if (identical(parameter.kind, ElementKind.FIELD_PARAMETER)) {
             if (parameter.name.slowToString() == nameAsString) {
               tryMakeMemberPlaceholder(name);
               break;
@@ skipping 220 matching lines @@
 
   visitBlock(Block node) {
     for (Node statement in node.statements.nodes) {
       if (statement is VariableDefinitions) {
         makeVarDeclarationTypePlaceholder(statement);
       }
     }
     node.visitChildren(this);
   }
 }