Implement 'analysis.getErrors' for the new driver.

pkg/analyzer/test/src/task/strong/strong_test_helper.dart

Index: pkg/analyzer/test/src/task/strong/strong_test_helper.dart
diff --git a/pkg/analyzer/test/src/task/strong/strong_test_helper.dart b/pkg/analyzer/test/src/task/strong/strong_test_helper.dart
index fd162c8b883f13b2653d1346d44da05ea2e5a530..c2debb8bddcbe088e9e8f2b7c568c668e6c33777 100644
--- a/pkg/analyzer/test/src/task/strong/strong_test_helper.dart
+++ b/pkg/analyzer/test/src/task/strong/strong_test_helper.dart
@@ -175,7 +175,9 @@ String _errorCodeName(ErrorCode errorCode) {
 
 ErrorSeverity _errorSeverity(AnalysisContext context, AnalysisError error) {
   // Attempt to process severity in a similar way to analyzer_cli and server.
-  return ErrorProcessor.getProcessor(context, error)?.severity ??
+  return ErrorProcessor
+          .getProcessor(context.analysisOptions, error)
+          ?.severity ??
       error.errorCode.errorSeverity;
 }