Use a "jsonrpc" field rather than "json-rpc" in the VM service.

runtime/vm/json_stream.cc

Index: runtime/vm/json_stream.cc
diff --git a/runtime/vm/json_stream.cc b/runtime/vm/json_stream.cc
index 9a3054b3d83034286ddc7c3e8019d568132fd31b..96dfb0d605d2d0c6d4da01921c03e772adc65b86 100644
--- a/runtime/vm/json_stream.cc
+++ b/runtime/vm/json_stream.cc
@@ -74,13 +74,13 @@ void JSONStream::Setup(Zone* zone,
               isolate_name, method_);
     setup_time_micros_ = OS::GetCurrentTimeMicros();
   }
-  buffer_.Printf("{\"json-rpc\":\"2.0\", \"result\":");
+  buffer_.Printf("{\"jsonrpc\":\"2.0\", \"result\":");
 }
 
 
 void JSONStream::SetupError() {
   buffer_.Clear();
-  buffer_.Printf("{\"json-rpc\":\"2.0\", \"error\":");
+  buffer_.Printf("{\"jsonrpc\":\"2.0\", \"error\":");
 }