Introduce CompletionTarget to take the place of the Node being completed.

pkg/analyzer/lib/src/generated/element_resolver.dart

 // Copyright (c) 2014, 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.
 
 library engine.resolver.element_resolver;
 
 import 'dart:collection';
 
 import 'ast.dart';
 import 'element.dart';
@@ skipping 3082 matching lines @@
    */
   SyntheticIdentifier(this.name, this.targetIdentifier);
 
   @override
   sc.Token get beginToken => null;
 
   @override
   Element get bestElement => null;
 
   @override
+  Iterable get childEntities {
+    // Should never be called, since a SyntheticIdentifier never appears in the
+    // AST--it is just used for lookup.
+    assert(false);
+    return new ChildEntities();
+  }
+
+  @override
   sc.Token get endToken => null;
 
   @override
   int get length => targetIdentifier.length;
 
   @override
   int get offset => targetIdentifier.offset;
 
   @override
   int get precedence => 16;
 
   @override
   Element get propagatedElement => null;
 
   @override
   Element get staticElement => null;
 
   @override
   accept(AstVisitor visitor) => null;
 
   @override
   void visitChildren(AstVisitor visitor) {
   }
 }