Revert "Report errors on duplicate export."

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

Index: sdk/lib/_internal/compiler/implementation/library_loader.dart
diff --git a/sdk/lib/_internal/compiler/implementation/library_loader.dart b/sdk/lib/_internal/compiler/implementation/library_loader.dart
index 6a247e209c633f2df7c821a03215fdccb00968a8..359aa435f4b7bc7a141e8c8eb74715c9f462d1a0 100644
--- a/sdk/lib/_internal/compiler/implementation/library_loader.dart
+++ b/sdk/lib/_internal/compiler/implementation/library_loader.dart
@@ -173,12 +173,11 @@ class LibraryLoaderTask extends LibraryLoader {
     Uri base = library.entryCompilationUnit.script.uri;
     for (LibraryTag tag in library.tags.reverse()) {
       if (tag.isImport) {
-        Import import = tag;
-        tagState = checkTag(TagState.IMPORT_OR_EXPORT, import);
-        if (import.uri.dartString.slowToString() == 'dart:core') {
+        tagState = checkTag(TagState.IMPORT_OR_EXPORT, tag);
+        if (tag.uri.dartString.slowToString() == 'dart:core') {
           importsDartCore = true;
         }
-        libraryDependencies.addLast(import);
+        libraryDependencies.addLast(tag);
       } else if (tag.isExport) {
         tagState = checkTag(TagState.IMPORT_OR_EXPORT, tag);
         libraryDependencies.addLast(tag);
@@ -191,11 +190,10 @@ class LibraryLoaderTask extends LibraryLoader {
         }
         checkDuplicatedLibraryName(library);
       } else if (tag.isPart) {
-        Part part = tag;
-        StringNode uri = part.uri;
+        StringNode uri = tag.uri;
         Uri resolved = base.resolve(uri.dartString.slowToString());
-        tagState = checkTag(TagState.SOURCE, part);
-        scanPart(part, resolved, library);
+        tagState = checkTag(TagState.SOURCE, tag);
+        scanPart(tag, resolved, library);
       } else {
         compiler.internalError("Unhandled library tag.", node: tag);
       }
@@ -548,16 +546,8 @@ class LibraryDependencyNode {
     SourceString name = element.name;
     Element existingElement = exportScope[name];
     if (existingElement != null) {
-      if (existingElement.isErroneous()) {
-        compiler.reportMessage(compiler.spanFromElement(element),
-            MessageKind.DUPLICATE_EXPORT.error([name]), api.Diagnostic.ERROR);
-        element = existingElement;
-      } else if (existingElement.getLibrary() != library) {
+      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);
       }