Move FixProcessorErrorCodeTest.diagnosticCodeFilter

Only one test implementation used it, and it can be a proper
function instead of a getter that returns a function.

Work towards #55660

Change-Id: I9f6e6d4c6f88c0b613c69abec6b714ef5fe4bf64
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/426401
Auto-Submit: Samuel Rawlins <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>

Author: srawlins

pkg/analysis_server/test/src/services/correction/fix/fix_processor.dart

@@ -302,10 +302,6 @@ abstract class FixInFileProcessorTest extends BaseFixProcessorTest {
 abstract class FixProcessorErrorCodeTest extends FixProcessorTest {
   /// The diagnostic code being tested.
   DiagnosticCode get diagnosticCode;
-
-  ErrorFilter get diagnosticCodeFilter => (AnalysisError e) {
-    return e.errorCode == diagnosticCode;
-  };
 }
 
 /// A base class defining support for writing fix processor tests that are

pkg/analysis_server/test/src/services/correction/fix/merge_combinators_test.dart

@@ -554,6 +554,10 @@ import 'other.dart' show FutureOr, Completer, Timer;
 }
 
 abstract class _MergeCombinatorTest extends FixProcessorErrorCodeTest {
+  bool diagnosticCodeFilter(AnalysisError e) {
+    return e.errorCode == diagnosticCode;
+  }
+
   @override
   void setUp() {
     super.setUp();