Add data-URI support class to dart:core (next to Uri).

CHANGELOG.md

 ## 1.14.0
 
 ### Core library changes
 * `dart:math`
   * `Random` added a `secure` constructor returning a cryptographically secure
     random generator which reads from the entropy source provided by the
     embedder for every generated random value.
 
+* `dart:core`
+  * Added `Uri.data` getter for `data:` URIs, and `UriData` class for the
+    return type.
+
+* `dart:convert`
+  * `Base64Decoder.convert` now takes optional `start` and `end` parameters.
+
 ## 1.13.0
 
 ### Core library changes
 * `dart:async`
   * `StreamController` added getters for `onListen`, `onPause`, and `onResume`
     with the corresponding new `typedef void ControllerCallback()`.
   * `StreamController` added a getter for `onCancel` with the corresponding
     new `typedef ControllerCancelCallback()`;
   * `StreamTransformer` instances created with `fromHandlers` with no
     `handleError` callback now forward stack traces along with errors to the
@@ skipping 603 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.