Test for the case when the watch function is null.

pkg/front_end/test/incremental_kernel_generator_test.dart

 // Copyright (c) 2017, 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 'package:front_end/compiler_options.dart';
 import 'package:front_end/incremental_kernel_generator.dart';
 import 'package:front_end/memory_file_system.dart';
 import 'package:front_end/src/incremental/byte_store.dart';
@@ skipping 350 matching lines @@
     {
       DeltaProgram delta = await incrementalKernelGenerator.computeDelta();
       Program program = delta.newProgram;
       _assertLibraryUris(program,
           includes: [aUri, bUri, cUri], excludes: [Uri.parse('dart:core')]);
       // Compiled: c.dart (changed), and b.dart (has mixin).
       _assertCompiledUris([cUri, bUri]);
     }
   }
 
-  void _assertCompiledUris(Iterable<Uri> expected) {
-    var compiledCycles = incrementalKernelGenerator.test.compiledCycles;
-    Set<Uri> compiledUris = compiledCycles
-        .map((cycle) => cycle.libraries.map((file) => file.uri))
-        .expand((uris) => uris)
-        .toSet();
-    expect(compiledUris, unorderedEquals(expected));
-  }
-
   test_compile_typedef() async {
     writeFile('/test/.packages', 'test:lib/');
     String aPath = '/test/lib/a.dart';
     String bPath = '/test/lib/b.dart';
     writeFile(aPath, 'typedef int F<T>(T x);');
     Uri bUri = writeFile(
         bPath,
         r'''
 import 'a.dart';
 F<String> f;
@@ skipping 425 matching lines @@
     {
       await incrementalKernelGenerator.computeDelta();
       // No new used files.
       expect(usedFiles, isEmpty);
       // The file b.dart is not used anymore.
       expect(unusedFiles, [bUri]);
       unusedFiles.clear();
     }
   }
 
+  test_watch_null() async {
+    writeFile('/test/.packages', 'test:lib/');
+    String aPath = '/test/lib/a.dart';
+    String bPath = '/test/lib/b.dart';
+    Uri aUri = writeFile(aPath, "");
+    Uri bUri = writeFile(bPath, "");
+
+    // Set null, as if the watch function is not provided.
+    watchFn = null;
+
+    await getInitialState(bUri);
+
+    // Update b.dart to import a.dart file.
+    writeFile(bPath, "import 'a.dart';");
+    incrementalKernelGenerator.invalidate(bUri);
+    await incrementalKernelGenerator.computeDelta();
+
+    // No exception even though the watcher function is null.
+  }
+
   /// Write the given [text] of the file with the given [path] into the
   /// virtual filesystem.  Return the URI of the file.
   Uri writeFile(String path, String text) {
     Uri uri = Uri.parse('file://$path');
     fileSystem.entityForUri(uri).writeAsStringSync(text);
     return uri;
   }
 
   /// Write the given file contents to the virtual filesystem.
   void writeFiles(Map<String, String> contents) {
     contents.forEach(writeFile);
   }
 
+  void _assertCompiledUris(Iterable<Uri> expected) {
+    var compiledCycles = incrementalKernelGenerator.test.compiledCycles;
+    Set<Uri> compiledUris = compiledCycles
+        .map((cycle) => cycle.libraries.map((file) => file.uri))
+        .expand((uris) => uris)
+        .toSet();
+    expect(compiledUris, unorderedEquals(expected));
+  }
+
   void _assertLibraryUris(Program program,
       {List<Uri> includes: const [], List<Uri> excludes: const []}) {
     List<Uri> libraryUris =
         program.libraries.map((library) => library.importUri).toList();
     for (var shouldInclude in includes) {
       expect(libraryUris, contains(shouldInclude));
     }
     for (var shouldExclude in excludes) {
       expect(libraryUris, isNot(contains(shouldExclude)));
     }
   }
 
   Library _getLibrary(Program program, Uri uri) {
     for (var library in program.libraries) {
       if (library.importUri == uri) return library;
     }
     throw fail('No library found with URI "$uri"');
   }
 
   String _getLibraryText(Library library) {
     StringBuffer buffer = new StringBuffer();
     new Printer(buffer, syntheticNames: new NameSystem())
         .writeLibraryFile(library);
     return buffer.toString();
   }
 }