Include a "jsonrpc" property in all VM service responses.

R=johnmccutchan@google.com

Review URL: https://codereview.chromium.org//1258733004 .

CHANGELOG.md

@@ -76,6 +76,11 @@
 
 [package spec proposal]: https://github.com/lrhn/dep-pkgspec
 
+### VM Service Protocol Changes
+
+* Some RPCs that didn't include a `"jsonrpc"` property in their responses now
+  include one.
+
 ## 1.11.2
 
 ### Core library changes

runtime/vm/service/message.dart

@@ -143,6 +143,7 @@ class Message {
 
   void setErrorResponse(String message) {
     var response = {
+      'jsonrpc': '2.0',
       'id': serial,
       'result' : {
         'type': 'Error',

runtime/vm/service/vmservice.dart

@@ -156,6 +156,7 @@ class VMService extends MessageRouter {
 
   String _encodeError(Message message, int code, {String details}) {
     var response = {
+      'jsonrpc': '2.0',
       'id' : message.serial,
       'error' : {
         'code': code,
@@ -172,6 +173,7 @@ class VMService extends MessageRouter {
 
   String _encodeResult(Message message, Map result) {
     var response = {
+      'jsonrpc': '2.0',
       'id' : message.serial,
       'result' : result,
     };