Set MethodInvocation.interfaceTarget during type inference.

pkg/front_end/testcases/inference/downwards_inference_on_generic_function_expressions.dart

 // Copyright (c) 2017, 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.
 
 /*@testedFeatures=inference*/
 library test;
 
 void main() {
   {
     String f<S>(int x) => null;
@@ skipping 39 matching lines @@
         /*error:LIST_ELEMENT_TYPE_NOT_ASSIGNABLE*/ 3
       ];
     };
   }
   {
     int int2int<S>(int x) => null;
     String int2String<T>(int x) => null;
     String string2String<T>(String x) => null;
     var /*@type=int2int(int) -> int*/ x = int2int;
     x = /*@returnType=int*/ <T>(/*@type=int*/ x) => /*@promotedType=none*/ x;
-    x = /*@returnType=int*/ <T>(/*@type=int*/ x) => /*@promotedType=none*/ x +
-        1;
+    x = /*@returnType=int*/ <
+        T>(/*@type=int*/ x) => /*@promotedType=none*/ x /*@target=num::+*/ + 1;
     var /*@type=int2String(int) -> String*/ y = int2String;
     y = /*info:INFERRED_TYPE_CLOSURE, error:INVALID_ASSIGNMENT*/ /*@returnType=int*/ <
             T>(/*@type=int*/ x) =>
         /*@promotedType=none*/ x;
     y = /*@returnType=String*/ <
         T>(/*@type=int*/ x) => /*info:DYNAMIC_INVOKE, info:DYNAMIC_CAST*/ /*@promotedType=none*/ x.substring(3);
     var /*@type=string2String(String) -> String*/ z = string2String;
-    z = /*@returnType=String*/ <
-        T>(/*@type=String*/ x) => /*@promotedType=none*/ x.substring(3);
+    z = /*@returnType=String*/ <T>(/*@type=String*/ x) => /*@promotedType=none*/ x
+        . /*@target=String::substring*/ substring(3);
   }
 }