Refactor isolate startup code in preparation for making isolate spawning more truly non-blocking.

runtime/vm/port.h

Index: runtime/vm/port.h
diff --git a/runtime/vm/port.h b/runtime/vm/port.h
index 5a0f177402b5ef2f02a276fe5c286feb78eaca86..f62ac8bd6eefbc933bd009870a252512c867dd17 100644
--- a/runtime/vm/port.h
+++ b/runtime/vm/port.h
@@ -20,12 +20,18 @@ class PortMapTestPeer;
 
 class PortMap: public AllStatic {
  public:
+  enum PortState {
+    kNewPort = 0,      // a newly allocated port
+    kLivePort = 1,     // a regular port (has a ReceivePort)
+    kControlPort = 2,  // a special control port (has a ReceivePort)
+  };
+
   // Allocate a port for the provided handler and return its VM-global id.
   static Dart_Port CreatePort(MessageHandler* handler);
 
   // Indicates that a port has had a ReceivePort created for it at the
   // dart language level.  The port remains live until it is closed.
-  static void SetLive(Dart_Port id);
+  static void SetPortState(Dart_Port id, PortState kind);
 
   // Close the port with id. All pending messages will be dropped.
   //
@@ -59,9 +65,11 @@ class PortMap: public AllStatic {
   typedef struct {
     Dart_Port port;
     MessageHandler* handler;
-    bool live;
+    PortState state;
   } Entry;
 
+  static const char* PortStateString(PortState state);
+
   // Allocate a new unique port.
   static Dart_Port AllocatePort();