Capture more errors and improve duplicate error removal

pkg/analyzer/lib/src/generated/error.dart

 // Copyright (c) 2014, 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.
 
 library engine.error;
 
 import 'dart:collection';
 
 import 'ast.dart' show AstNode;
 import 'element.dart';
@@ skipping 191 matching lines @@
     //buffer.write("(" + lineNumber + ":" + columnNumber + "): ");
     buffer.write(_message);
     return buffer.toString();
   }
 
   /**
    * Merge all of the errors in the lists in the given list of [errorLists] into
    * a single list of errors.
    */
   static List<AnalysisError> mergeLists(List<List<AnalysisError>> errorLists) {
-    List<AnalysisError> errors = <AnalysisError>[];
+    Set<AnalysisError> errors = new HashSet<AnalysisError>();
     for (List<AnalysisError> errorList in errorLists) {
       errors.addAll(errorList);
     }
-    return errors;
+    return errors.toList();
   }
 }
 
 /**
  * An object that listen for [AnalysisError]s being produced by the analysis
  * engine.
  */
 abstract class AnalysisErrorListener {
   /**
    * An error listener that ignores errors that are reported to it.
@@ skipping 4711 matching lines @@
    * Initialize a newly created error code to have the given [name].
    */
   const TodoCode(String name) : super(name, "{0}");
 
   @override
   ErrorSeverity get errorSeverity => ErrorSeverity.INFO;
 
   @override
   ErrorType get type => ErrorType.TODO;
 }