[dtd] Improve the error thrown when trying to call a service method without a complete name

This now behaves the same as for any other method that doesn't exist, rather than throwing a generic RPC error with a `RangeError` inside.

Fixes https://github.com/dart-lang/sdk/issues/56540

Change-Id: I1171fe1c0c2a4a6f56d7a653af87b17a76af9d68
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/381643
Reviewed-by: Elliott Brooks <elliottbrooks@google.com>
Reviewed-by: Ben Konyi <bkonyi@google.com>
Commit-Queue: Ben Konyi <bkonyi@google.com>

pkg/dtd_impl/lib/src/dtd_client.dart

@@ -311,6 +311,14 @@ class DTDClient extends Client {
     // method, forward the request to that client.
     final combinedName = parameters.method;
     final dotIndex = combinedName.indexOf('.');
+    if (dotIndex == -1) {
+      // All service methods must have a dot in the name.
+      throw json_rpc.RpcException(
+        RpcErrorCodes.kMethodNotFound,
+        'Unknown service method: $combinedName',
+      );
+    }
+
     final serviceName = combinedName.substring(0, dotIndex);
     final methodName = combinedName.substring(dotIndex + 1);
 

pkg/dtd_impl/test/dtd_test.dart

@@ -330,6 +330,21 @@ void main() {
         );
       });
 
+      test('calling a method without a dot', () {
+        expect(
+          () => client.sendRequest('abc', {}),
+          throwsA(
+            isA<RpcException>()
+                .having(
+                  (e) => e.code,
+                  'code',
+                  RpcException.methodNotFound('abc').code,
+                )
+                .having((e) => e.message, 'message', 'Unknown method "abc".'),
+          ),
+        );
+      });
+
       test('different clients cannot register the same service', () async {
         final client2 = _createClient(uri);
         final registerResult = await client.sendRequest('registerService', {