Handle deferred access as pre-step in SemanticSendVisitor.

pkg/compiler/lib/src/elements/modelx.dart

 // Copyright (c) 2013, 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 elements.modelx;
 
 import 'common.dart';
 import 'elements.dart';
 import '../constants/expressions.dart';
 import '../constants/constructors.dart';
@@ skipping 1102 matching lines @@
     importScope.addImport(this, element, import, listener);
   }
 
   accept(ElementVisitor visitor, arg) {
     return visitor.visitPrefixElement(this, arg);
   }
 
   void markAsDeferred(Import deferredImport) {
     _deferredImport = deferredImport;
   }
+
+  String toString() => '$kind($name)';
 }
 
 class TypedefElementX extends ElementX
     with AstElementMixin,
          AnalyzableElementX,
          TypeDeclarationElementX<TypedefType>
     implements TypedefElement {
   Typedef cachedNode;
 
   /**
@@ skipping 100 matching lines @@
         message: "Constant has not been computed for $this."));
     return constantCache;
   }
 
   void set constant(ConstantExpression value) {
     if (isPatch) {
       ConstantVariableMixin originVariable = origin;
       originVariable.constant = value;
       return null;
     }
-    assert(invariant(this, constantCache == null || constantCache == value,
-        message: "Constant has already been computed for $this."));
+    assert(invariant(
+        this, constantCache == null || constantCache == value,
+        message: "Constant has already been computed for $this. "
+                 "Existing constant: ${constantCache.getText()}, "
+                 "New constant: ${value != null ? value.getText() : ''}."));
     constantCache = value;
   }
 }
 
 abstract class VariableElementX extends ElementX
     with AstElementMixin, ConstantVariableMixin
     implements VariableElement {
   final Token token;
   final VariableList variables;
   VariableDefinitions definitionsCache;
@@ skipping 1637 matching lines @@
   AstElement get definingElement;
 
   bool get hasResolvedAst => definingElement.hasTreeElements;
 
   ResolvedAst get resolvedAst {
     return new ResolvedAst(declaration,
         definingElement.node, definingElement.treeElements);
   }
 
 }