Add missing Observatory sources to gypi and re-sort.

runtime/observatory/observatory_sources.gypi

Index: runtime/observatory/observatory_sources.gypi
diff --git a/runtime/observatory/observatory_sources.gypi b/runtime/observatory/observatory_sources.gypi
index 19ba5cb5db8dbbbabaa84ca8cd3212c7d0f50919..a469c27a5705afdaa5c8ca69976c57429196570d 100644
--- a/runtime/observatory/observatory_sources.gypi
+++ b/runtime/observatory/observatory_sources.gypi
@@ -170,6 +170,7 @@
     'lib/src/elements/script_ref.dart',
     'lib/src/elements/script_view.dart',
     'lib/src/elements/script_view.html',
+    'lib/src/elements/sentinel_value.dart',
     'lib/src/elements/service_ref.dart',
     'lib/src/elements/service_ref.html',
     'lib/src/elements/service_view.dart',
@@ -201,6 +202,7 @@
     'lib/src/models/objects/flag.dart',
     'lib/src/models/objects/frame.dart',
     'lib/src/models/objects/function.dart',
+    'lib/src/models/objects/guarded.dart',
     'lib/src/models/objects/heap_space.dart',
     'lib/src/models/objects/icdata.dart',
     'lib/src/models/objects/instance.dart',
@@ -231,6 +233,7 @@
     'lib/src/models/repositories/sample_profile.dart',
     'lib/src/models/repositories/script.dart',
     'lib/src/models/repositories/target.dart',
+    'lib/src/repositories/allocation_profile.dart',
     'lib/src/repositories/class.dart',
     'lib/src/repositories/event.dart',
     'lib/src/repositories/flag.dart',