Support JSON-RPC 2.0-compatible VM service events.

CHANGELOG.md

 ## 1.12.0
 
 ### Core library changes
 
 * `dart:async`
   * `StreamController`  added setters for the `onListen`, `onPause`, `onResume`
     and `onCancel` callbacks.
 
 * `dart:convert`
   * `LineSplitter` added a `split` static method returning an `Iterable`.
@@ skipping 63 matching lines @@
     * A stack overflow caused by a transformer being run multiple times on the
       package that defines it has been fixed.
 
     * A transformer that tries to read a non-existent asset in another package
       will now be re-run if that asset is later created.
 
 [package spec proposal]: https://github.com/lrhn/dep-pkgspec
 
 ### VM Service Protocol Changes
 
+* **BREAKING** The service protocol now sends JSON-RPC 2.0-compatible
+  server-to-client events.
+
 * The service protocol now includes a `"jsonrpc"` property in its responses, as
   opposed to `"json-rpc"`.
 
 * The service protocol now properly handles requests with non-string ids.
   Numeric ids are no longer converted to strings, and null ids now don't produce
   a response.
 
 * Some RPCs that didn't include a `"jsonrpc"` property in their responses now
   include one.
 
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     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.