[dart2dart] Do not crash on unresolved named constructor

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 123 matching lines @@
   visitTypeReferenceSend(Send node) {
     collector.makeElementPlaceholder(node.selector, elements[node]);
   }
 }
 
 class PlaceholderCollector extends Visitor {
   final Compiler compiler;
   final Set<String> fixedMemberNames; // member names which cannot be renamed.
   final Map<Element, ElementAst> elementAsts;
   final Set<Node> nullNodes;  // Nodes that should not be in output.
-  final Set<Identifier> unresolvedNodes;
+  final Set<Node> unresolvedNodes;
   final Map<Element, Set<Node>> elementNodes;
   final Map<FunctionElement, FunctionScope> functionScopes;
   final Map<LibraryElement, Set<Identifier>> privateNodes;
   final List<DeclarationTypePlaceholder> declarationTypePlaceholders;
   final Map<String, Set<Identifier>> memberPlaceholders;
   final Map<Element, List<ConstructorPlaceholder>> constructorPlaceholders;
   Map<String, LocalPlaceholder> currentLocalPlaceholders;
   Element currentElement;
   FunctionElement topmostEnclosingFunction;
   TreeElements treeElements;
 
   LibraryElement get coreLibrary => compiler.coreLibrary;
   FunctionElement get entryFunction => compiler.mainApp.find(Compiler.MAIN);
 
   get currentFunctionScope => functionScopes.putIfAbsent(
       topmostEnclosingFunction, () => new FunctionScope());
 
   PlaceholderCollector(this.compiler, this.fixedMemberNames, this.elementAsts) :
       nullNodes = new Set<Node>(),
-      unresolvedNodes = new Set<Identifier>(),
+      unresolvedNodes = new Set<Node>(),
       elementNodes = new Map<Element, Set<Node>>(),
       functionScopes = new Map<FunctionElement, FunctionScope>(),
       privateNodes = new Map<LibraryElement, Set<Identifier>>(),
       declarationTypePlaceholders = new List<DeclarationTypePlaceholder>(),
       memberPlaceholders = new Map<String, Set<Identifier>>(),
       constructorPlaceholders =
           new Map<Element, List<ConstructorPlaceholder>>();
 
   void collectFunctionDeclarationPlaceholders(
       FunctionElement element, FunctionExpression node) {
@@ skipping 430 matching lines @@
 
   visitBlock(Block node) {
     for (Node statement in node.statements.nodes) {
       if (statement is VariableDefinitions) {
         makeVarDeclarationTypePlaceholder(statement);
       }
     }
     node.visitChildren(this);
   }
 }