Perform incremental resolution only if the existing unit is not null.

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 'ast.dart';
@@ skipping 1168 matching lines @@
     _resolveApiChanges = resolveApiChanges;
   }
 
   /**
    * Attempts to update [oldUnit] to the state corresponding to [newCode].
    * Returns `true` if success, or `false` otherwise.
    * The [oldUnit] might be damaged.
    */
   bool resolve(CompilationUnit oldUnit, String newCode) {
     logger.enter('diff/resolve $_unitSource');
-    logger.log(oldUnit != null ? 'has oldUnit' : 'oldUnit is null');
     try {
       _unitElement = oldUnit.element;
       CompilationUnit newUnit = _parseUnit(newCode);
       _TokenPair firstPair =
           _findFirstDifferentToken(oldUnit.beginToken, newUnit.beginToken);
       _TokenPair lastPair =
           _findLastDifferentToken(oldUnit.endToken, newUnit.endToken);
       if (firstPair != null && lastPair != null) {
         int firstOffsetOld = firstPair.oldToken.offset;
         int firstOffsetNew = firstPair.newToken.offset;
@@ skipping 591 matching lines @@
   String toString() => name;
 }
 
 
 class _TokenPair {
   final _TokenDifferenceKind kind;
   final Token oldToken;
   final Token newToken;
   _TokenPair(this.kind, this.oldToken, this.newToken);
 }