Avoid using Invalid{Statement,Expression,Initializer}.

pkg/compiler/lib/src/kernel/fasta_support.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.
 
 /// Additions to Fasta for generating .dill (Kernel IR) files with dart2js patch
 /// files and native hooks.
 library compiler.src.kernel.fasta_support;
 
 // TODO(sigmund): get rid of this file. Fasta should be agnostic of the
 // target platform, at which point this should not be necessary. In particular,
@@ skipping 72 matching lines @@
   KernelTargetForDart2js(
       DillTarget target, TranslateUri uriTranslator, bool strongMode,
       [Map<String, Source> uriToSource])
       : super(PhysicalFileSystem.instance, target, uriTranslator, strongMode,
             uriToSource);
 
   @override
   Token skipNativeClause(Token token) => _skipNative(token);
 
   @override
-  String extractNativeMethodName(Token token) => null;
+  String extractNativeMethodName(Token token) => "";
 
   @override
   void loadExtraRequiredLibraries(Loader loader) => _loadExtras(loader);
 
   @override
   void runBuildTransformations() {}
 }
 
 /// Specializes [DillTarget] to build kernel for dart2js: JS-specific libraries
 /// are included in the SDK, and native clauses have no string parameter.
 class DillTargetForDart2js extends DillTarget {
   DillTargetForDart2js(Ticker ticker, TranslateUri uriTranslator)
       : super(ticker, uriTranslator, "none");
 
   @override
   Token skipNativeClause(Token token) => _skipNative(token);
 
   @override
-  String extractNativeMethodName(Token token) => null;
+  String extractNativeMethodName(Token token) => "";
 
   @override
   void loadExtraRequiredLibraries(Loader loader) => _loadExtras(loader);
 }
 
 /// We use native clauses of this form in our dart2js patch files:
 ///
 ///     methodDeclaration() native;
 ///
 /// The default front_end parser doesn't support this, so it will trigger an
@@ skipping 32 matching lines @@
   'dart:html_common',
   'dart:indexed_db',
   'dart:js',
   'dart:js_util',
   'dart:mirrors',
   'dart:svg',
   'dart:web_audio',
   'dart:web_gl',
   'dart:web_sql',
 ];