Add "not intended to be implemented/extended" warnings to pkg/front_end.

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';
 
 /// Callback used to report errors encountered during compilation.
 typedef void ErrorHandler(CompilationError error);
 
 /// Front-end options relevant to compiler back ends.
 ///
 /// TODO(paulberry): add a mechanism to allow file system operations to be
 /// stubbed out for testing.
+///
+/// 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.
@@ skipping 56 matching lines @@
   ///
   /// The value should be a power of two, and should match the `PLATFORM` bit
   /// flags in sdk/lib/_internal/sdk_library_metadata/lib/libraries.dart.  If
   /// zero, no patch files will be applied.
   int platformBit;
 
   /// The declared variables for use by configurable imports and constant
   /// evaluation.
   Map<String, String> declaredVariables;
 }