Add isolate tag-profile and better handling of errors

runtime/vm/service/running_isolates.dart

Index: runtime/vm/service/running_isolates.dart
diff --git a/runtime/vm/service/running_isolates.dart b/runtime/vm/service/running_isolates.dart
index 7f6b045cb40277888f88511bf952ee4c04e1a016..8c7bc81abfcf7090f669af1134f922d0315d2433 100644
--- a/runtime/vm/service/running_isolates.dart
+++ b/runtime/vm/service/running_isolates.dart
@@ -6,15 +6,22 @@ part of vmservice;
 
 class RunningIsolates implements MessageRouter {
   final Map<int, RunningIsolate> isolates = new Map<int, RunningIsolate>();
+  int _rootPortId;
 
   RunningIsolates();
 
   void isolateStartup(int portId, SendPort sp, String name) {
+    if (_rootPortId == null) {
+      _rootPortId = portId;
+    }
     var ri = new RunningIsolate(portId, sp, name);
     isolates[portId] = ri;
   }
 
   void isolateShutdown(int portId, SendPort sp) {
+    if (_rootPortId == portId) {
+      _rootPortId = null;
+    }
     isolates.remove(portId);
   }
 
@@ -32,12 +39,17 @@ class RunningIsolates implements MessageRouter {
       return message.response;
     }
     var isolateId;
-    try {
-      isolateId = int.parse(message.path[1]);
-    } catch (e) {
-      message.setErrorResponse('Could not parse isolate id: $e');
-      return message.response;
+    if ((message.path[1] == 'root') && (_rootPortId != null)) {
+      isolateId = _rootPortId;

[turnidge, 2014/03/24 20:22:13]

so isolates/root is an alias to whatever the root isolate's id is?

Not sure that this works so well, post my change.  Whenever we see an isolate we
don't recognize, we reload the VM rather than just trusting the id and reloading
the isolate.  You may want to drop this feature and we can add it later.

Another option is to suport "root" in the Observatory itself.

[Cutch, 2014/03/25 14:39:06]

This alias will never been seen by Observatory, it's primarily useful for
testing from the command line. I'd like to keep this in for now.

+    } else {
+      try {
+        isolateId = int.parse(message.path[1]);
+      } catch (e) {
+        message.setErrorResponse('Could not parse isolate id: $e');
+        return message.response;
+      }
     }
+    assert(isolateId != null);
     var isolate = isolates[isolateId];
     if (isolate == null) {
       message.setErrorResponse('Cannot find isolate id: $isolateId');