Add type inference for complex assignments whose LHS is an index expression.

pkg/front_end/testcases/inference/infer_assign_to_property_custom.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;
 
 class A {
   int operator +(other) => 1;
   double operator -(other) => 2.0;
 }
 
 class B {
   A a;
 }
 
-var /*@topType=dynamic*/ v_prefix_pp = (/*@target=A::+*/ ++new B()
-    . /*error:TOP_LEVEL_INSTANCE_GETTER*/ /*@target=B::a*/ a);
-var /*@topType=dynamic*/ v_prefix_mm = (/*@target=A::-*/ --new B()
-    . /*error:TOP_LEVEL_INSTANCE_GETTER*/ /*@target=B::a*/ a);
+var /*@topType=dynamic*/ v_prefix_pp =
+    (++new B(). /*error:TOP_LEVEL_INSTANCE_GETTER*/ /*@target=B::a*/ a);
+var /*@topType=dynamic*/ v_prefix_mm =
+    (--new B(). /*error:TOP_LEVEL_INSTANCE_GETTER*/ /*@target=B::a*/ a);
 var /*@topType=dynamic*/ v_postfix_pp =
     (new B(). /*error:TOP_LEVEL_INSTANCE_GETTER*/ /*@target=B::a*/ a++);
 var /*@topType=dynamic*/ v_postfix_mm =
     (new B(). /*error:TOP_LEVEL_INSTANCE_GETTER*/ /*@target=B::a*/ a--);
 
 main() {}