Added CHANGELOG.md entry on syntax-only generic methods.

CHANGELOG.md

 ## 1.21.0
 
 ### Language
 
+* Support generic methods with erasure. This feature does not introduce
+  full-fledged support for generic methods, but it bridges the gap to a
+  full implementation. It allows for declaring type parameters on methods,
+  passing actual type arguments in function and method invocations, and
+  getting static checks from the analyzer. The missing part is reification
+  of type arguments at runtime. For details please check the
+  [informal specification](https://gist.github.com/eernstg/4353d7b4f669745bed3a5423e04a453c).

[Lasse Reichstein Nielsen, 2016/11/28 08:55:45]

I think this is somewhat too wordy for a CHANGES entry. Can we make it short and
crisp? Don't say what it doesn't do, don't say what it might do later, just say
what we have right now.

Maybe something like:

* Allow *writing* generic functions and function calls. The type arguments are
removed at runtime, but the analyzer is allowed to use them for warnings. This
allows writing strong-mode compatible code that can also be used by spec-mode
programs. 
See [informal
specification](https://gist.github.com/eernstg/4353d7b4f669745bed3a5423e04a453c)
for more information.

 * 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.
 
@@ skipping 1357 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.