update changelog to mention pub support for dartdevc

CHANGELOG.md

 ## 1.24.0
 
 ### Language
 * During a dynamic type check, `void` is not required to be `null` anymore.
   In practice, this makes overriding `void` functions with non-`void` functions
   safer.
 * During static analysis, a function or setter declared using `=>` with return
   type `void` now allows the returned expression to have any type. For example,
   assuming the declaration `int x;`, it is now type correct to have
   `void f() => ++x;`.
@@ skipping 58 matching lines @@
 * `dart:async`, `dart:io`, `dart:core`
     * Adding to a closed sink, including `IOSink`, is not allowed anymore. In
       1.24, violations are only reported (on stdout or stderr), but a future
       version of the Dart SDK will change this to throwing a `StateError`.
 
 ### Dart VM
 
 ### Tool Changes
 
 * Pub
+    * Added support for the Dart Development Compiler. There are two ways of
+      opting in:

[nweiz, 2017/05/25 19:54:13]

Mention which pub commands are affected.

[nweiz, 2017/05/25 19:54:13]

Nit: it would read easier in plain text to add some whitespace between the
different bullet points here.

[jakemac, 2017/05/25 21:35:36]

Done.

[jakemac, 2017/05/25 21:35:36]

Done.

+        * Use the new `--web-compiler` flag, which supports `dartdevc`,
+          `dart2js` or `none` as options. This is the easiest way to try things
+          out without changing the default.

[nweiz, 2017/05/25 19:54:13]

Provide some more detail here about what exact behavior this affects. Remember,
this is the only documentation people have for this for the crucial time period
where they can try it out and suggest breaking changes.

[jakemac, 2017/05/25 21:35:36]

Done.

+        * Add config to your pubspec. There is a new `web` key which supports a
+          single key called `compiler`. This is a map of string to string, where
+          the keys are modes and the values are the compiler to use in that
+          mode. For example, to default to dartdevc in debug mode you can add
+          the following to your pubspec:
+
+          ```yaml
+          web:
+            compiler:
+              debug: dartdevc
+          ```

[nweiz, 2017/05/25 19:54:13]

It's probably best to wait to mention this until it's landed in the SDK.

[jakemac, 2017/05/25 21:35:36]

I was just going to wait to merge this until it is in the sdk, updating it more
than once seems redundant (and also the flag names here would need to change).

     * `pub build` will use a failing exit code if there are errors in any
       transformer.
     * Allow publishing packages that depend on the Flutter SDK.
 
 * dartfmt
     * Preserve type parameters in new generic function typedef syntax.
     * Add self-test validation to ensure formatter bugs do not cause user code
       to be lost.
 
 ### Infrastructure changes
@@ skipping 1853 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.