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

pkg/front_end/lib/compiler_options.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library front_end.compiler_options;
 
 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);
 
 /// Front-end options relevant to compiler back ends.
 ///
 /// Not intended to be implemented or extended by clients.
 class CompilerOptions {
   /// The path to the Dart SDK.
   ///
   /// If `null`, the SDK will be searched for using
   /// [Platform.resolvedExecutable] as a starting point.
   ///
   /// This option is mutually exclusive with [sdkSummary].
   String sdkPath;
 
   /// 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.
   ///
   /// If `null`, the ".packages" file will be found via the standard
   /// package_config search algorithm.
   String packagesFilePath;
 
   /// Paths to the input summary files (excluding the SDK summary).  These files
   /// should all be linked summaries.  They should also be closed, in the sense
   /// that any libraries they reference should also appear in [inputSummaries]
@@ skipping 50 matching lines @@
   Map<String, String> declaredVariables;
 
   /// The [FileSystem] which should be used by the front end to access files.
   ///
   /// All file system access performed by the front end goes through this
   /// mechanism, with one exception: if no value is specified for
   /// [packagesFilePath], the packages file is located using the actual physical
   /// file system.  TODO(paulberry): fix this.
   FileSystem fileSystem = PhysicalFileSystem.instance;
 }