Create a default error handler to avoid the need for null checks.

pkg/front_end/lib/compiler_options.dart

Index: pkg/front_end/lib/compiler_options.dart
diff --git a/pkg/front_end/lib/compiler_options.dart b/pkg/front_end/lib/compiler_options.dart
index ac75dd112906137788f7eb2640441b359c901a9e..05be3eaaff8e2c8eeaf33042d23cadf3c5c2f303 100644
--- a/pkg/front_end/lib/compiler_options.dart
+++ b/pkg/front_end/lib/compiler_options.dart
@@ -8,6 +8,9 @@ import 'compilation_error.dart';
 import 'file_system.dart';
 import 'physical_file_system.dart';
 
+/// Default error handler used by [CompielerOptions.onError].
+void defaultErrorHandler(CompilationError error) => throw error;
+
 /// Callback used to report errors encountered during compilation.
 typedef void ErrorHandler(CompilationError error);
 
@@ -25,9 +28,9 @@ class CompilerOptions {
 
   /// Callback to which compilation errors should be delivered.
   ///
-  /// If `null`, the first error will be reported by throwing an exception of
+  /// By default, the first error will be reported by throwing an exception of
   /// type [CompilationError].
-  ErrorHandler onError;
+  ErrorHandler onError = defaultErrorHandler;
 
   /// Path to the ".packages" file.
   ///