Use Zone to correctly measure async operations.

pkg/compiler/lib/src/apiimpl.dart

 // Copyright (c) 2012, 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.
 
 library leg_apiimpl;
 
 import 'dart:async';
 import 'dart:convert';
 
 import '../compiler_new.dart' as api;
@@ skipping 468 matching lines @@
       });
     } catch (ex, s) {
       diagnoseCrashInUserCode(
           'Uncaught exception in diagnostic handler', ex, s);
       rethrow;
     }
   }
 
   Future callUserProvider(Uri uri) {
     try {
-      return userProviderTask.measure(() => provider.readFromUri(uri));
+      return userProviderTask.measureIo(() => provider.readFromUri(uri));
     } catch (ex, s) {
       diagnoseCrashInUserCode('Uncaught exception in input provider', ex, s);
       rethrow;
     }
   }
 
   Future<Packages> callUserPackagesDiscovery(Uri uri) {
     try {
-      return userPackagesDiscoveryTask.measure(
+      return userPackagesDiscoveryTask.measureIo(
                  () => packagesDiscoveryProvider(uri));
     } catch (ex, s) {
       diagnoseCrashInUserCode('Uncaught exception in package discovery', ex, s);
       rethrow;
     }
   }
 
   void diagnoseCrashInUserCode(String message, exception, stackTrace) {
     hasCrashed = true;
     print('$message: ${tryToString(exception)}');
     print(tryToString(stackTrace));
   }
 
   fromEnvironment(String name) => environment[name];
 
   library_info.LibraryInfo lookupLibraryInfo(String libraryName) {
     return library_info.libraries[libraryName];
   }
 }