If there is a changed file, hasFilesToAnalyze is true.

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

 // Copyright (c) 2016, 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.
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:typed_data';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart' show CompilationUnitElement;
@@ skipping 256 matching lines @@
 
   /**
    * The current file system state.
    */
   FileSystemState get fsState => _fsState;
 
   /**
    * Return `true` if the driver has a file to analyze.
    */
   bool get hasFilesToAnalyze {
-    return _requestedFiles.isNotEmpty ||
+    return _changedFiles.isNotEmpty ||
+        _requestedFiles.isNotEmpty ||
         _requestedParts.isNotEmpty ||
         _filesToAnalyze.isNotEmpty ||
         _partsToAnalyze.isNotEmpty;
   }
 
   /**
    * Return the set of files that are known at this moment. This set does not
    * always include all added files or all implicitly used file. If a file has
    * not been processed yet, it might be missing.
    */
@@ skipping 1306 matching lines @@
           libraryDeclarations.add(new TopLevelDeclarationInSource(
               file.source, declaration, isExported));
         }
       }
     }
 
     // We're not done yet.
     return false;
   }
 }