Abort tracing in type inference in the presence of await.

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

Index: pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
diff --git a/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart b/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
index 730466152cc209303d720755402bb4ed002ab1c8..2b63c0c7d61c5e7939722eac8b3cebaae9f6f773 100644
--- a/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
+++ b/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
@@ -1128,6 +1128,14 @@ class TypeGraphInferrerEngine
     return info;
   }
 
+  TypeInformation registerAwait(ast.Node node, TypeInformation argument) {
+    AwaitTypeInformation info = new AwaitTypeInformation(types.currentMember,
+                                                         node);
+    info.addAssignment(argument);
+    types.allocatedTypes.add(info);
+    return info;
+  }
+
   TypeInformation registerCalledClosure(ast.Node node,
                                         Selector selector,
                                         TypeInformation closure,