Model 'await for' as an explicit node in tree and fix its handling.

pkg/compiler/lib/src/inferrer/type_graph_nodes.dart

Index: pkg/compiler/lib/src/inferrer/type_graph_nodes.dart
diff --git a/pkg/compiler/lib/src/inferrer/type_graph_nodes.dart b/pkg/compiler/lib/src/inferrer/type_graph_nodes.dart
index b87a8926bfad57eeb64b8c3348f49eb8d9886b2c..dac18f95c18ebd24cebef6be65468743df024a75 100644
--- a/pkg/compiler/lib/src/inferrer/type_graph_nodes.dart
+++ b/pkg/compiler/lib/src/inferrer/type_graph_nodes.dart
@@ -674,7 +674,7 @@ class StaticCallSiteTypeInformation extends CallSiteTypeInformation {
   }
 
   bool get isSynthesized {
-    // Some calls do not have a corresponding node, for example
+    // Some calls do not have a corresponding selector, for example
     // fowarding factory constructors, or synthesized super
     // constructor calls. We synthesize these calls but do
     // not create a selector for them.