Avoid using Invalid{Statement,Expression,Initializer}.

pkg/compiler/lib/src/kernel/fasta_support.dart

Index: pkg/compiler/lib/src/kernel/fasta_support.dart
diff --git a/pkg/compiler/lib/src/kernel/fasta_support.dart b/pkg/compiler/lib/src/kernel/fasta_support.dart
index af3b833499166340dc5fbbff5f0f6be69122e314..16fc8da72ee23c46d95721e794a00c90eb48beaf 100644
--- a/pkg/compiler/lib/src/kernel/fasta_support.dart
+++ b/pkg/compiler/lib/src/kernel/fasta_support.dart
@@ -90,7 +90,7 @@ class KernelTargetForDart2js extends KernelTarget {
   Token skipNativeClause(Token token) => _skipNative(token);
 
   @override
-  String extractNativeMethodName(Token token) => null;
+  String extractNativeMethodName(Token token) => "";
 
   @override
   void loadExtraRequiredLibraries(Loader loader) => _loadExtras(loader);
@@ -109,7 +109,7 @@ class DillTargetForDart2js extends DillTarget {
   Token skipNativeClause(Token token) => _skipNative(token);
 
   @override
-  String extractNativeMethodName(Token token) => null;
+  String extractNativeMethodName(Token token) => "";
 
   @override
   void loadExtraRequiredLibraries(Loader loader) => _loadExtras(loader);