Report errors on duplicate export.

sdk/lib/_internal/compiler/implementation/library_loader.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.
 
 part of dart2js;
 
 /**
  * [CompilerTask] for loading libraries and setting up the import/export scopes.
  */
 abstract class LibraryLoader extends CompilerTask {
@@ skipping 155 matching lines @@
         return tagState;
       }
       return TagState.NEXT[value];
     }
 
     bool importsDartCore = false;
     var libraryDependencies = new LinkBuilder<LibraryDependency>();
     Uri base = library.entryCompilationUnit.script.uri;
     for (LibraryTag tag in library.tags.reverse()) {
       if (tag.isImport) {
-        tagState = checkTag(TagState.IMPORT_OR_EXPORT, tag);
-        if (tag.uri.dartString.slowToString() == 'dart:core') {
+        Import import = tag;
+        tagState = checkTag(TagState.IMPORT_OR_EXPORT, import);
+        if (import.uri.dartString.slowToString() == 'dart:core') {
           importsDartCore = true;
         }
-        libraryDependencies.addLast(tag);
+        libraryDependencies.addLast(import);
       } else if (tag.isExport) {
         tagState = checkTag(TagState.IMPORT_OR_EXPORT, tag);
         libraryDependencies.addLast(tag);
       } else if (tag.isLibraryName) {
         tagState = checkTag(TagState.LIBRARY, tag);
         if (library.libraryTag != null) {
           compiler.cancel("duplicated library declaration", node: tag);
         } else {
           library.libraryTag = tag;
         }
         checkDuplicatedLibraryName(library);
       } else if (tag.isPart) {
-        StringNode uri = tag.uri;
+        Part part = tag;
+        StringNode uri = part.uri;
         Uri resolved = base.resolve(uri.dartString.slowToString());
-        tagState = checkTag(TagState.SOURCE, tag);
-        scanPart(tag, resolved, library);
+        tagState = checkTag(TagState.SOURCE, part);
+        scanPart(part, resolved, library);
       } else {
         compiler.internalError("Unhandled library tag.", node: tag);
       }
     }
 
     // Apply patch, if any.
     if (library.uri.scheme == 'dart') {
       patchDartLibrary(handler, library, library.uri.path);
     }
 
@@ skipping 332 matching lines @@
   }
 
   /**
    * Adds [element] to the export scope for this node. If the [element] name
    * is a duplicate, an error element is inserted into the export scope.
    */
   Element addElementToExportScope(Compiler compiler, Element element) {
     SourceString name = element.name;
     Element existingElement = exportScope[name];
     if (existingElement != null) {
-      if (existingElement.getLibrary() != library) {
+      if (existingElement.isErroneous()) {
+        compiler.reportMessage(compiler.spanFromElement(element),
+            MessageKind.DUPLICATE_EXPORT.error([name]), api.Diagnostic.ERROR);
+        element = existingElement;
+      } else if (existingElement.getLibrary() != library) {
         // Declared elements hide exported elements.
+        compiler.reportMessage(compiler.spanFromElement(existingElement),
+            MessageKind.DUPLICATE_EXPORT.error([name]), api.Diagnostic.ERROR);
+        compiler.reportMessage(compiler.spanFromElement(element),
+            MessageKind.DUPLICATE_EXPORT.error([name]), api.Diagnostic.ERROR);
         element = exportScope[name] = new ErroneousElement(
             MessageKind.DUPLICATE_EXPORT, [name], name, library);
       }
     } else {
       exportScope[name] = element;
     }
     return element;
   }
 
   /**
@@ skipping 129 matching lines @@
   }
 
   /**
    * Registers all top-level entities of [library] as starting point for the
    * fixed-point computation of the import/export scopes.
    */
   void registerLibraryExports(LibraryElement library) {
     nodeMap[library].registerInitialExports();
   }
 }