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

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

Index: pkg/analyzer/lib/src/generated/engine.dart
diff --git a/pkg/analyzer/lib/src/generated/engine.dart b/pkg/analyzer/lib/src/generated/engine.dart
index 7db8f36fa88d19723c909a2c550c83ce58992084..26ef329a55ec0f099f398e59a26d13ee711fe412 100644
--- a/pkg/analyzer/lib/src/generated/engine.dart
+++ b/pkg/analyzer/lib/src/generated/engine.dart
@@ -2343,8 +2343,8 @@ class AnalysisContextImpl implements InternalAnalysisContext {
           // 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()
@@ -2377,8 +2377,8 @@ class AnalysisContextImpl implements InternalAnalysisContext {
             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 &&
@@ -2918,6 +2918,12 @@ class AnalysisContextImpl implements InternalAnalysisContext {
               _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 =