Print the location of duplicated elements.

dart/sdk/lib/_internal/compiler/implementation/elements/elements.dart

Index: dart/sdk/lib/_internal/compiler/implementation/elements/elements.dart
diff --git a/dart/sdk/lib/_internal/compiler/implementation/elements/elements.dart b/dart/sdk/lib/_internal/compiler/implementation/elements/elements.dart
index 3d7982b191f17fd7909a0e599a38c726e250ab87..4bed073adc4c281328a1a09f86aac046b1e78e0a 100644
--- a/dart/sdk/lib/_internal/compiler/implementation/elements/elements.dart
+++ b/dart/sdk/lib/_internal/compiler/implementation/elements/elements.dart
@@ -253,6 +253,8 @@ abstract class Element implements Spannable {
   /// 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 {
@@ -626,7 +628,7 @@ abstract class LibraryElement extends Element implements ScopeContainerElement {
 
   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).