Shift AnalysisError instances only once during incremental resolution.

pkg/analyzer/lib/src/generated/incremental_resolver.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.incremental_resolver;
 
 import 'dart:collection';
 import 'dart:math' as math;
 
 import 'package:analyzer/src/context/cache.dart'
@@ skipping 878 matching lines @@
    */
   final int _updateEndOld;
 
   /**
    * The end of the changed contents in the new unit.
    */
   final int _updateEndNew;
 
   int _updateDelta;
 
+  /**
+   * The set of [AnalysisError]s that have been already shifted.
+   */
+  Set<AnalysisError> _alreadyShiftedErrors = new HashSet.identity();
+
   RecordingErrorListener errorListener = new RecordingErrorListener();
   ResolutionContext _resolutionContext;
 
   List<AnalysisError> _resolveErrors = AnalysisError.NO_ERRORS;
   List<AnalysisError> _verifyErrors = AnalysisError.NO_ERRORS;
 
   /**
    * Initialize a newly created incremental resolver to resolve a node in the
    * given source in the given library.
    */
@@ skipping 230 matching lines @@
 
   void _shiftEntryErrors_OLD() {
     _shiftErrors_OLD(DartEntry.RESOLUTION_ERRORS);
     _shiftErrors_OLD(DartEntry.VERIFICATION_ERRORS);
     _shiftErrors_OLD(DartEntry.HINTS);
     _shiftErrors_OLD(DartEntry.LINTS);
   }
 
   void _shiftErrors(List<AnalysisError> errors) {
     for (AnalysisError error in errors) {
-      int errorOffset = error.offset;
-      if (errorOffset > _updateOffset) {
-        error.offset += _updateDelta;
+      if (_alreadyShiftedErrors.add(error)) {
+        int errorOffset = error.offset;
+        if (errorOffset > _updateOffset) {
+          error.offset += _updateDelta;
+        }
       }
     }
   }
 
   void _shiftErrors_NEW(ResultDescriptor<List<AnalysisError>> descriptor) {
     List<AnalysisError> errors = newUnitEntry.getValue(descriptor);
     _shiftErrors(errors);
   }
 
   void _shiftErrors_OLD(DataDescriptor<List<AnalysisError>> descriptor) {
@@ skipping 862 matching lines @@
   @override
   String toString() => name;
 }
 
 class _TokenPair {
   final _TokenDifferenceKind kind;
   final Token oldToken;
   final Token newToken;
   _TokenPair(this.kind, this.oldToken, this.newToken);
 }