Clarify in dev_compiler docs the reason for the angular whitelist hack

pkg/dev_compiler/lib/src/compiler/compiler.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.
 
 import 'dart:collection' show HashSet, Queue;
 import 'dart:convert' show JSON;
 import 'dart:io' show File;
 
 import 'package:analyzer/analyzer.dart'
     show AnalysisError, CompilationUnit, ErrorSeverity;
@@ skipping 248 matching lines @@
 
   /// Hoist types from class signatures
   final bool hoistSignatureTypes;
 
   /// Name types in type tests
   final bool nameTypeTests;
 
   /// Hoist types in type tests
   final bool hoistTypeTests;
 
+  // TODO(kevmoo): Remove once https://github.com/dart-lang/sdk/issues/27255
+  //               is fixed.
   final bool useAngular2Whitelist;
 
   /// Enable ES6 destructuring of named parameters. Off by default.
   ///
   /// Older V8 versions do not accept default values with destructuring in
   /// arrow functions yet (e.g. `({a} = {}) => 1`) but happily accepts them
   /// with regular functions (e.g. `function({a} = {}) { return 1 }`).
   ///
   /// Supporting the syntax:
   /// * Chrome Canary (51)
@@ skipping 89 matching lines @@
           defaultsTo: true,
           hide: hide)
       ..addFlag('hoist-signature-types',
           help: 'Hoist types from class signatures',
           defaultsTo: false,
           hide: hide)
       ..addFlag('name-type-tests',
           help: 'Name types used in type tests', defaultsTo: true, hide: hide)
       ..addFlag('hoist-type-tests',
           help: 'Hoist types used in type tests', defaultsTo: true, hide: hide)
+      // TODO(kevmoo): Remove once https://github.com/dart-lang/sdk/issues/27255
+      //               is fixed.
       ..addFlag('unsafe-angular2-whitelist', defaultsTo: false, hide: hide)
       ..addOption('bazel-mapping',
           help:
               '--bazel-mapping=genfiles/to/library.dart,to/library.dart uses \n'
               'to/library.dart as the path for library.dart in source maps.',
           allowMultiple: true,
           splitCommas: false,
           hide: hide)
       ..addOption('summary-out',
           help: 'location to write the summary file', hide: hide);
@@ skipping 213 matching lines @@
 /// Strip out files that should not be included in the sdk sourcemap as they
 /// are implementation details that would just confuse users.
 /// Normalize sdk urls to use "dart:" for more understandable stack traces.
 Map cleanupSdkSourcemap(Map sourceMap) {
   var map = new Map.from(sourceMap);
   map['sources'] = map['sources']
       .map((url) => url.contains('/_internal/') ? null : url)
       .toList();
   return map;
 }