Issue 29968. When a file is added, and it is already known, refresh it.

pkg/analyzer/lib/src/dart/analysis/driver.dart

Index: pkg/analyzer/lib/src/dart/analysis/driver.dart
diff --git a/pkg/analyzer/lib/src/dart/analysis/driver.dart b/pkg/analyzer/lib/src/dart/analysis/driver.dart
index da2436d23b5d0b430474827a8d6d7a3ae07b7245..f95375865fe3d4f43fd4ba3321b9de0adb4ad9c4 100644
--- a/pkg/analyzer/lib/src/dart/analysis/driver.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/driver.dart
@@ -480,6 +480,13 @@ class AnalysisDriver implements AnalysisDriverGeneric {
     }
     if (AnalysisEngine.isDartFileName(path)) {
       _fileTracker.addFile(path);
+      // If the file is known, it has already been read, even if it did not
+      // exist. Now we are notified that the file exists, so we need to
+      // re-read it and make sure that we invalidate signature of the files
+      // that reference it.
+      if (_fsState.knownFilePaths.contains(path)) {
+        _changeFile(path);
+      }
     }
   }
 
@@ -503,8 +510,7 @@ class AnalysisDriver implements AnalysisDriverGeneric {
    */
   void changeFile(String path) {
     _throwIfChangesAreNotAllowed();
-    _fileTracker.changeFile(path);
-    _priorityResults.clear();
+    _changeFile(path);
   }
 
   /**
@@ -947,6 +953,14 @@ class AnalysisDriver implements AnalysisDriverGeneric {
   }
 
   /**
+   * Implementation for [changeFile].
+   */
+  void _changeFile(String path) {
+    _fileTracker.changeFile(path);
+    _priorityResults.clear();
+  }
+
+  /**
    * Handles a notification from the [FileTracker] that there has been a change
    * of state.
    */