Add dart:developer.postEvent for posting events to the service protocol from Dart code

runtime/observatory/tests/service/developer_extension_test.dart

Index: runtime/observatory/tests/service/developer_extension_test.dart
diff --git a/runtime/observatory/tests/service/developer_extension_test.dart b/runtime/observatory/tests/service/developer_extension_test.dart
index 1fa04bd263a0fd313642ca4beb8e1646a599caa5..e29686bc0a68eb5b0cbba3a98b82c07e65c52d25 100644
--- a/runtime/observatory/tests/service/developer_extension_test.dart
+++ b/runtime/observatory/tests/service/developer_extension_test.dart
@@ -56,6 +56,10 @@ Future<ServiceExtensionResponse> LanguageErrorHandler(String method,
 void test() {
   registerExtension('__delay', Handler);
   debugger();
+  postEvent('ALPHA', {
+    'cat': 'dog'
+  });
+  debugger();
   registerExtension('__error', Handler);
   registerExtension('__exception', Handler);
   registerExtension('__null', Handler);
@@ -78,6 +82,13 @@ var tests = [
     expect(isolate.extensionRPCs.length, 1);
     expect(isolate.extensionRPCs[0], equals('__delay'));
   },
+  resumeIsolateAndAwaitEvent(Isolate.kExtensionStream, (ServiceEvent event) {
+    expect(event.kind, equals(ServiceEvent.kExtension));
+    expect(event.extensionKind, equals('ALPHA'));
+    expect(event.extensionData, new isInstanceOf<Map>());
+    expect(event.extensionData['cat'], equals('dog'));
+  }),
+  hasStoppedAtBreakpoint,
   resumeIsolateAndAwaitEvent(VM.kIsolateStream, (ServiceEvent event) {
     // Check that we received an event when '__error' was registered.
     expect(event.kind, equals(ServiceEvent.kServiceExtensionAdded));