Add an IsolateRunnable event to the service protocol. Improve service docs.

runtime/vm/service_event.cc

Index: runtime/vm/service_event.cc
diff --git a/runtime/vm/service_event.cc b/runtime/vm/service_event.cc
index ce1e7fb04905a4ee12dc7a551482ee1e9e1e6100..9c9868a933e6e5b484e1711fa3d29fbaea05daac 100644
--- a/runtime/vm/service_event.cc
+++ b/runtime/vm/service_event.cc
@@ -62,6 +62,8 @@ const char* ServiceEvent::KindAsCString() const {
   switch (kind()) {
     case kIsolateStart:
       return "IsolateStart";
+    case kIsolateRunnable:
+      return "IsolateRunnable";
     case kIsolateExit:
       return "IsolateExit";
     case kIsolateUpdate:
@@ -106,6 +108,7 @@ const char* ServiceEvent::KindAsCString() const {
 const char* ServiceEvent::stream_id() const {
   switch (kind()) {
     case kIsolateStart:
+    case kIsolateRunnable:
     case kIsolateExit:
     case kIsolateUpdate:
       return Service::isolate_stream.id();