Print the location of duplicated elements.

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

Index: dart/sdk/lib/_internal/compiler/implementation/compiler.dart
diff --git a/dart/sdk/lib/_internal/compiler/implementation/compiler.dart b/dart/sdk/lib/_internal/compiler/implementation/compiler.dart
index f71d6e61513d2cbaa4c02e0e0aad32836bdd939e..671b02a5ff90feac2055f8dc4d618b835f0a239f 100644
--- a/dart/sdk/lib/_internal/compiler/implementation/compiler.dart
+++ b/dart/sdk/lib/_internal/compiler/implementation/compiler.dart
@@ -1094,6 +1094,13 @@ abstract class Compiler implements DiagnosticListener {
                   api.Diagnostic.WARNING);
   }
 
+  void reportInfo(Spannable node, MessageKind errorCode,
+                  [Map arguments = const {}]) {
+    reportMessage(spanFromSpannable(node),
+                  errorCode.error(arguments),
+                  api.Diagnostic.INFO);
+  }
+
   void reportMessage(SourceSpan span, Diagnostic message, api.Diagnostic kind) {
     // TODO(ahe): The names Diagnostic and api.Diagnostic are in
     // conflict. Fix it.