Allow null as a synonym for {} when decoding JSON objects.

pkg/analysis_server/tool/spec/codegen_dart_protocol.dart

Index: pkg/analysis_server/tool/spec/codegen_dart_protocol.dart
diff --git a/pkg/analysis_server/tool/spec/codegen_dart_protocol.dart b/pkg/analysis_server/tool/spec/codegen_dart_protocol.dart
index cf0388cfe1a5c777fa80feab6b69c88f40fb18b1..bf2cbb3878ca811c3be8266e5231fab577ee7112 100644
--- a/pkg/analysis_server/tool/spec/codegen_dart_protocol.dart
+++ b/pkg/analysis_server/tool/spec/codegen_dart_protocol.dart
@@ -538,6 +538,11 @@ class CodegenProtocolVisitor extends HierarchicalApiVisitor with CodeGenerator {
         'factory $className.fromJson(JsonDecoder jsonDecoder, String jsonPath, Object json) {'
         );
     indent(() {
+      writeln('if (json == null) {');
+      indent(() {
+        writeln('json = {};');
+      });
+      writeln('}');
       writeln('if (json is Map) {');
       indent(() {
         List<String> args = <String>[];