Remove DebuggerEvent. Refactor remaining code.

runtime/vm/service.h

Index: runtime/vm/service.h
diff --git a/runtime/vm/service.h b/runtime/vm/service.h
index f38f7f8352824d0c2a9acc83e5cd9be382dbdf67..084b2180031f5c78b588eb7d12a67cdbfa030f46 100644
--- a/runtime/vm/service.h
+++ b/runtime/vm/service.h
@@ -66,10 +66,10 @@ class StreamInfo {
  public:
   explicit StreamInfo(const char* id) : id_(id), enabled_(false) {}
 
-  const char* id() { return id_; }
+  const char* id() const { return id_; }
 
   void set_enabled(bool value) { enabled_ = value; }
-  bool enabled() { return enabled_; }
+  bool enabled() const { return enabled_; }
 
  private:
   const char* id_;