update docs on dart:developer Service class

CHANGELOG.md

 ## 1.21.0
 
 ### Language
 
 * Support generic method syntax. Type arguments are not available at
   runtime. For details, check the
   [informal specification](https://gist.github.com/eernstg/4353d7b4f669745bed3a5423e04a453c).
 * Support access to initializing formals, e.g., the use of `x` to initialize
  `y` in `class C { var x, y; C(this.x): y = x; }`.
   Please check the
   [informal specification](https://gist.github.com/eernstg/cff159be9e34d5ea295d8c24b1a3e594)
   for details.
 * Don't warn about switch case fallthrough if the case ends in a `rethrow`
   statement.
 * Also don't warn if the entire switch case is wrapped in braces - as long as
   the block ends with a `break`, `continue`, `rethrow`, `return` or `throw`.
 * Allow `=` as well as `:` as separator for named parameter default values.
 
 ### Core library changes
 
 * `dart:core`: `Set.difference` now takes a `Set<Object>` as argument.
 
-* `dart:developer`:
-  * The service protocol http server can now be controlled from Dart code.
+* `dart:developer`
+
+  * Added `Service` class.
+    * Allows inspecting and controlling the VM service protocol HTTP server.
+    * Provides an API to access the ID of an `Isolate`.
 
 ### Tool changes
 
 * Dart Dev Compiler
 
   * Support calls to `loadLibrary()` on deferred libraries. Deferred libraries
     are still loaded eagerly. (#27343)
 
 ## 1.20.1 - 2016-10-13
 
@@ skipping 1345 matching lines @@
     they will keep the Dart process alive until they time out. This fixes the
     handling of persistent connections. Previously, the client would shut down
     immediately after a request.
 
   * **Breaking change:** `HttpServer` no longer compresses all traffic by
     default. The new `autoCompress` property can be set to `true` to re-enable
     compression.
 
 * `dart:isolate`: `Isolate.spawnUri` added the optional `packageRoot` argument,
   which controls how it resolves `package:` URIs.