When the same contents is set for a source, we still need to reset its modification time.

pkg/analyzer/lib/src/generated/engine.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.
 
 // This code was auto-generated, is not intended to be edited, and is subject to
 // significant change. Please see the README file for more information.
 
 library engine;
 
 import "dart:math" as math;
@@ skipping 2325 matching lines @@
     bool hintsEnabled = _options.hint;
     MapIterator<Source, SourceEntry> iterator = _cache.iterator();
     while (iterator.moveNext()) {
       Source source = iterator.key;
       SourceEntry sourceEntry = iterator.value;
       for (DataDescriptor descriptor in sourceEntry.descriptors) {
         if (descriptor == DartEntry.SOURCE_KIND) {
           // The source kind is always valid, so the state isn't interesting.
           continue;
         } else if (descriptor == DartEntry.CONTAINING_LIBRARIES) {
-          // The list of containing libraries is always valid, so the state isn't
-          // interesting.
+          // The list of containing libraries is always valid, so the state
+          // isn't interesting.
           continue;
         } else if (descriptor == DartEntry.PUBLIC_NAMESPACE) {
           // The public namespace isn't computed by performAnalysisTask()
           // and therefore isn't interesting.
           continue;
         } else if (descriptor == HtmlEntry.ANGULAR_APPLICATION ||
             descriptor == HtmlEntry.ANGULAR_COMPONENT ||
             descriptor == HtmlEntry.ANGULAR_ENTRY ||
             descriptor == HtmlEntry.ANGULAR_ERRORS ||
             descriptor == HtmlEntry.POLYMER_BUILD_ERRORS ||
@@ skipping 12 matching lines @@
             sourceEntry.getState(descriptor));
       }
       if (sourceEntry is DartEntry) {
         // get library-specific values
         List<Source> librarySources = getLibrariesContaining(source);
         for (Source librarySource in librarySources) {
           for (DataDescriptor descriptor in sourceEntry.libraryDescriptors) {
             if (descriptor == DartEntry.ANGULAR_ERRORS ||
                 descriptor == DartEntry.BUILT_ELEMENT ||
                 descriptor == DartEntry.BUILT_UNIT) {
-              // These values are not currently being computed, so their state is
-              // not interesting.
+              // These values are not currently being computed, so their state
+              // is not interesting.
               continue;
             } else if (source.isInSystemLibrary &&
                 !_generateSdkErrors &&
                 (descriptor == DartEntry.VERIFICATION_ERRORS ||
                     descriptor == DartEntry.HINTS)) {
               continue;
             } else if (!hintsEnabled && descriptor == DartEntry.HINTS) {
               continue;
             }
             callback(
@@ skipping 519 matching lines @@
             !_tryPoorMansIncrementalResolution(source, contents)) {
           _sourceChanged(source);
         }
         changed = true;
         SourceEntry sourceEntry = _cache.get(source);
         if (sourceEntry != null) {
           sourceEntry.modificationTime =
               _contentCache.getModificationStamp(source);
           sourceEntry.setValue(SourceEntry.CONTENT, contents);
         }
+      } else {
+        SourceEntry sourceEntry = _cache.get(source);
+        if (sourceEntry != null) {
+          sourceEntry.modificationTime =
+              _contentCache.getModificationStamp(source);
+        }
       }
     } else if (originalContents != null) {
       _incrementalAnalysisCache =
           IncrementalAnalysisCache.clear(_incrementalAnalysisCache, source);
       _sourceChanged(source);
       changed = true;
     }
     return changed;
   }
 
@@ skipping 12000 matching lines @@
 
   @override
   Object visitPolymerTagDartElement(PolymerTagDartElement element) {
     if (element.name == PolymerHtmlUnitBuilder_this._elementName) {
       throw new PolymerHtmlUnitBuilder_FoundTagDartElementError(
           element as PolymerTagDartElementImpl);
     }
     return null;
   }
 }