Add an intrinsified early out path for Dart timeline calls

sdk/lib/developer/timeline.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart.developer;
 
 const bool _isProduct = const bool.fromEnvironment("dart.vm.product");
 
 typedef dynamic TimelineSyncFunction();
 typedef Future TimelineAsyncFunction();
 
 /// Add to the timeline.
 class Timeline {
   /// Start a synchronous operation labeled [name]. Optionally takes
   /// a [Map] of [arguments]. This operation must be finished before
   /// returning to the event queue.
   static void startSync(String name, {Map arguments}) {
     if (_isProduct) {
       return;
     }
     if (name is! String) {
       throw new ArgumentError.value(name,
                                     'name',
                                     'Must be a String');
     }
+    if (!_isDartStreamEnabled()) {
+      // Push a null onto the stack and return.
+      _stack.add(null);
+      return;
+    }
     var block = new _SyncBlock._(name, _getTraceClock(), _getThreadCpuClock());
     if (arguments is Map) {
       block._appendArguments(arguments);
     }
     _stack.add(block);
   }
 
   /// Finish the last synchronous operation that was started.
   static void finishSync() {
     if (_isProduct) {
       return;
     }
     if (_stack.length == 0) {
       throw new StateError(
           'Uneven calls to startSync and finishSync');
     }
     // Pop top item off of stack.
     var block = _stack.removeLast();
+    if (block == null) {
+      // Dart stream was disabled when startSync was called.
+      return;
+    }
     // Finish it.
     block.finish();
   }
 
   /// Emit an instant event.
   static void instantSync(String name, {Map arguments}) {
     if (_isProduct) {
       return;
     }
     if (name is! String) {
       throw new ArgumentError.value(name,
                                     'name',
                                     'Must be a String');
     }
+    if (!_isDartStreamEnabled()) {
+      // Stream is disabled.
+      return;
+    }
     Map instantArguments;
     if (arguments is Map) {
       instantArguments = new Map.from(arguments);
     }
     _reportInstantEvent(_getTraceClock(),
                         'Dart',
                         name,
                         _argumentsAsJson(instantArguments));
   }
 
@@ skipping 203 matching lines @@
     if (_fastPathArguments == null) {
       _fastPathArguments = '{"isolateNumber":"${Timeline._isolateId}"}';
     }
     return _fastPathArguments;
   }
   // Add isolateNumber to arguments map.
   arguments['isolateNumber'] = Timeline._isolateIdString;
   return JSON.encode(arguments);
 }
 
+/// Returns true if the Dart Timeline stream is enabled.
+external bool _isDartStreamEnabled();
+
 /// Returns the next async task id.
 external int _getNextAsyncId();
 
 /// Returns the current value from the trace clock.
 external int _getTraceClock();
 
 /// Returns the current value from the thread CPU usage clock.
 external int _getThreadCpuClock();
 
 /// Returns the isolate's main port number.
@@ skipping 12 matching lines @@
                                    int startCpu,
                                    String category,
                                    String name,
                                    String argumentsAsJson);
 
 /// Reports an instant event.
 external void _reportInstantEvent(int start,
                                   String category,
                                   String name,
                                   String argumentsAsJson);