Set MethodInvocation.interfaceTarget during type inference.

pkg/front_end/testcases/inference/downwards_inference_on_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;
 
 typedef T Function2<S, T>(S x);
 
 void main() {
@@ skipping 39 matching lines @@
     Function2<int, List<String>> l4 = /*@returnType=List<String>*/ (int x) {
       return /*@typeArgs=String*/ [
         /*error:LIST_ELEMENT_TYPE_NOT_ASSIGNABLE*/ 3
       ];
     };
   }
   {
     Function2<int, int>
         l0 = /*@returnType=int*/ (/*@type=int*/ x) => /*@promotedType=none*/ x;
     Function2<int, int>
-        l1 = /*@returnType=int*/ (/*@type=int*/ x) => /*@promotedType=none*/ x +
+        l1 = /*@returnType=int*/ (/*@type=int*/ x) => /*@promotedType=none*/ x /*@target=num::+*/ +
             1;
     Function2<int, String>
         l2 = /*error:INVALID_ASSIGNMENT*/ /*@returnType=int*/ (/*@type=int*/ x) =>
             /*@promotedType=none*/ x;
     Function2<int, String>
         l3 = /*@returnType=String*/ (/*@type=int*/ x) => /*info:DYNAMIC_CAST, info:DYNAMIC_INVOKE*/ /*@promotedType=none*/ x
             .substring(3);
     Function2<String, String>
         l4 = /*@returnType=String*/ (/*@type=String*/ x) => /*@promotedType=none*/ x
-            .substring(3);
+            . /*@target=String::substring*/ substring(3);
   }
 }