Begin migrating the vm service from a rest-style interface to a json-rpc style interface.

runtime/observatory/lib/service_html.dart

Index: runtime/observatory/lib/service_html.dart
diff --git a/runtime/observatory/lib/service_html.dart b/runtime/observatory/lib/service_html.dart
index da5a9759c43c68fc66202da1c02cd142852ba293..d51ef782bcfa8da9e8618abeafbdfae725817d17 100644
--- a/runtime/observatory/lib/service_html.dart
+++ b/runtime/observatory/lib/service_html.dart
@@ -100,4 +100,8 @@ class PostMessageVM extends VM {
     window.parent.postMessage(JSON.encode(message), '*');
     return completer.future;
   }
+
+  Future<String> invokeRpcRaw(String method, Map params) {
+    throw 'not-implemented';
+  }
 }