Update Changelog with conditional imports.

CHANGELOG.md

 ## 1.15.0
 
+### Language features
+* Added support for [configuration-specific imports](https://github.com/munificent/dep-interface-libraries/blob/master/Proposal.md).
+  On the VM and dart2js, they can be enabled with `--conditional-directives`.
+
+  The analyzer requires an additional option in an `options` file:
+  ```
+  analyzer:
+    language:
+      enableConditionalDirectives: true
+  ```
+  Then run the analyzer with `--options=<path-to-options-file>`.
+
 ### Core library changes
-  * Added `Uri.queryParametersAll` to handle multiple query parameters with
-    the same name.
+* Added `Uri.queryParametersAll` to handle multiple query parameters with
+  the same name.
 
 * `dart:io`
   * Added `SecurityContext.usePrivateKeyBytes`,
           `SecurityContext.useCertificateChainBytes`,
           `SecurityContext.setTrustedCertificatesBytes`, and
           `SecurityContext.setClientAuthoritiesBytes`.
   * The non-`Bytes` methods of `SecurityContext` are being renamed -`Sync`, as
     they will do synchronous IO. The non-`Bytes` and non-`Sync` methods are
     deprecated and will be removed in a later release.
   * **Breaking** The named `directory` argument of
@@ skipping 777 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.