Issue 6824. Re-export of conflicting elements is error.

compiler/java/com/google/dart/compiler/resolver/LibraryElementImplementation.java

Index: compiler/java/com/google/dart/compiler/resolver/LibraryElementImplementation.java
diff --git a/compiler/java/com/google/dart/compiler/resolver/LibraryElementImplementation.java b/compiler/java/com/google/dart/compiler/resolver/LibraryElementImplementation.java
index ea4d530e8a0311f862d5ffcac215969f2b653e27..baffee0fee103c6625239acd6f96ee29e2658e55 100644
--- a/compiler/java/com/google/dart/compiler/resolver/LibraryElementImplementation.java
+++ b/compiler/java/com/google/dart/compiler/resolver/LibraryElementImplementation.java
@@ -4,18 +4,18 @@
 
 package com.google.dart.compiler.resolver;
 
-import com.google.common.collect.Lists;
+import com.google.common.collect.Maps;
 import com.google.dart.compiler.ast.DartObsoleteMetadata;
 import com.google.dart.compiler.ast.LibraryUnit;
 
 import java.util.Collection;
-import java.util.List;
+import java.util.Map;
 
 class LibraryElementImplementation extends AbstractNodeElement implements LibraryElement {
 
   private final Scope importScope = new Scope("import", this);
   private final Scope scope = new Scope("library", this, importScope);
-  private final List<Element> exportedElements = Lists.newArrayList();
+  private final Map<String, Element> exportedElements = Maps.newHashMap();
   private LibraryUnit libraryUnit;
   private MethodElement entryPoint;
   private DartObsoleteMetadata metadata;
@@ -41,12 +41,13 @@ class LibraryElementImplementation extends AbstractNodeElement implements Librar
     return scope;
   }
 
-  public void addExportedElements(Element element) {
-    exportedElements.add(element);
+  public Element addExportedElements(Element element) {
+    String name = element.getName();
+    return exportedElements.put(name, element);
   }
   
-  public List<Element> getExportedElements() {
-    return exportedElements;
+  public Collection<Element> getExportedElements() {
+    return exportedElements.values();
   }
 
   @Override