Revert "Abort tracing in type inference in the presence of await."

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

Index: pkg/compiler/lib/src/inferrer/concrete_types_inferrer.dart
diff --git a/pkg/compiler/lib/src/inferrer/concrete_types_inferrer.dart b/pkg/compiler/lib/src/inferrer/concrete_types_inferrer.dart
index 46cd38607d3652f32995bf7207a88298c8a43236..4f952146517052a087a3c66a7363b3057c792484 100644
--- a/pkg/compiler/lib/src/inferrer/concrete_types_inferrer.dart
+++ b/pkg/compiler/lib/src/inferrer/concrete_types_inferrer.dart
@@ -2243,11 +2243,6 @@ class ConcreteTypesInferrer
     return result;
   }
 
-  ConcreteType registerAwait(Node node, ConcreteType argumentType) {
-    // TODO(polux): Properly handle await expressions.
-    return types.dynamicType;
-  }
-
   @override
   void setDefaultTypeOfParameter(ParameterElement parameter,
                                  ConcreteType type) {