Log more information to debug 'null' location for constructor.

pkg/analyzer/lib/src/generated/incremental_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.incremental_resolver;
 
 import 'dart:collection';
 import 'dart:math' as math;
 
 import 'ast.dart';
@@ skipping 167 matching lines @@
 
   @override
   visitConstructorDeclaration(ConstructorDeclaration node) {
     _hasConstructor = true;
     SimpleIdentifier constructorName = node.name;
     ConstructorElementImpl element = constructorName == null ?
         _enclosingClass.unnamedConstructor :
         _enclosingClass.getNamedConstructor(constructorName.name);
     _processElement(element);
     _assertCompatibleParameters(node.parameters, element.parameters);
+    // TODO(scheglov) debug null Location
+    if (element != null) {
+      if (element.context == null || element.source == null) {
+        logger.log('Bad constructor element $element for $node in ${node.parent}');
+      }
+    }
     // matches, update the existing element
     ExecutableElement newElement = node.element;
     node.element = element;
     _setLocalElements(element, newElement);
   }
 
   @override
   visitEnumConstantDeclaration(EnumConstantDeclaration node) {
     String name = node.name.name;
     FieldElement element = _findElement(_enclosingClass.fields, name);
@@ skipping 1515 matching lines @@
   String toString() => name;
 }
 
 
 class _TokenPair {
   final _TokenDifferenceKind kind;
   final Token oldToken;
   final Token newToken;
   _TokenPair(this.kind, this.oldToken, this.newToken);
 }