Clarify in dev_compiler docs the reason for the angular whitelist hack

pkg/dev_compiler/lib/src/compiler/compiler.dart

Index: pkg/dev_compiler/lib/src/compiler/compiler.dart
diff --git a/pkg/dev_compiler/lib/src/compiler/compiler.dart b/pkg/dev_compiler/lib/src/compiler/compiler.dart
index 0c61b7571d19fd95e10be0132009a4a19303a5b6..e90d37259c18e5796344975881da371aebaac522 100644
--- a/pkg/dev_compiler/lib/src/compiler/compiler.dart
+++ b/pkg/dev_compiler/lib/src/compiler/compiler.dart
@@ -266,6 +266,8 @@ class CompilerOptions {
   /// 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.
@@ -375,6 +377,8 @@ class CompilerOptions {
           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: