Print the location of duplicated elements.

dart/sdk/lib/_internal/compiler/implementation/elements/elements.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 elements;
 
 
 import 'modelx.dart';
 import '../tree/tree.dart';
 import '../util/util.dart';
@@ skipping 235 matching lines @@
   void addMetadata(MetadataAnnotation annotation);
   void setNative(String name);
   void setFixedBackendName(String name);
 
   Scope buildScope();
 
   /// If the element is a forwarding constructor, [targetConstructor] holds
   /// the generative constructor that the forwarding constructor points to
   /// (possibly via other forwarding constructors).
   FunctionElement get targetConstructor;
+
+  void diagnose(Element context, DiagnosticListener listener) {}
 }
 
 class Elements {
   static bool isUnresolved(Element e) {
     return e == null || e.isErroneous();
   }
   static bool isErroneousElement(Element e) => e != null && e.isErroneous();
 
   static bool isClass(Element e) => e != null && e.kind == ElementKind.CLASS;
   static bool isTypedef(Element e) {
@@ skipping 353 matching lines @@
   bool get exportsHandled;
 
   // TODO(kasperl): We should try to get rid of these.
   void set canUseNative(bool value);
   void set libraryTag(LibraryName value);
 
   LibraryElement get implementation;
 
   void addCompilationUnit(CompilationUnitElement element);
   void addTag(LibraryTag tag, DiagnosticListener listener);
-  void addImport(Element element, DiagnosticListener listener);
+  void addImport(Element element, Import import, DiagnosticListener listener);
 
   /// Record which element an import or export tag resolved to.
   /// (Belongs on builder object).
   void recordResolvedTag(LibraryDependency tag, LibraryElement library);
 
   /// Return the library element corresponding to an import or export.
   LibraryElement getLibraryFromTag(LibraryDependency tag);
 
   void addMember(Element element, DiagnosticListener listener);
   void addToScope(Element element, DiagnosticListener listener);
@@ skipping 272 matching lines @@
   int get resolutionState;
   Token get beginToken;
   Token get endToken;
 
   // TODO(kasperl): Try to get rid of these.
   void set annotatedElement(Element value);
   void set resolutionState(int value);
 
   MetadataAnnotation ensureResolved(Compiler compiler);
 }