Remove AST-hack for multi-field declarations.

pkg/compiler/lib/src/resolution/resolution.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.
 
 library dart2js.resolution;
 
 import 'dart:collection' show Queue;
 
 import '../common.dart';
 import '../common/names.dart' show Identifiers;
@@ skipping 342 matching lines @@
     return reporter.withCurrentElement(element, () {
       VariableDefinitions tree = element.parseNode(parsingContext);
       if (element.modifiers.isStatic && element.isTopLevel) {
         reporter.reportErrorMessage(element.modifiers.getStatic(),
             MessageKind.TOP_LEVEL_VARIABLE_DECLARED_STATIC);
       }
       ResolverVisitor visitor = visitorFor(element);
       ResolutionRegistry registry = visitor.registry;
       // TODO(johnniwinther): Maybe remove this when placeholderCollector migrates
       // to the backend ast.
-      registry.defineElement(tree.definitions.nodes.head, element);
+      registry.defineElement(element.definition, element);
       // TODO(johnniwinther): Share the resolved type between all variables
       // declared in the same declaration.
       if (tree.type != null) {
         DartType type = visitor.resolveTypeAnnotation(tree.type);
         assert(invariant(
             element,
             element.variables.type == null ||
                 // Crude check but we have no equivalence relation that
                 // equates malformed types, like matching creations of type
                 // `Foo<Unresolved>`.
@@ skipping 747 matching lines @@
   TreeElements get treeElements {
     assert(invariant(this, _treeElements != null,
         message: "TreeElements have not been computed for $this."));
     return _treeElements;
   }
 
   void reuseElement() {
     _treeElements = null;
   }
 }