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

pkg/front_end/testcases/inference/infer_assign_to_property.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 f;
 }
 
 var /*@topType=dynamic*/ v_assign =
     (/*error:TOP_LEVEL_UNSUPPORTED*/ new A(). /*@target=A::f*/ f = 1);
 var /*@topType=dynamic*/ v_plus =
     (/*error:TOP_LEVEL_UNSUPPORTED*/ new A(). /*@target=A::f*/ f += 1);
 var /*@topType=dynamic*/ v_minus =
     (/*error:TOP_LEVEL_UNSUPPORTED*/ new A(). /*@target=A::f*/ f -= 1);
 var /*@topType=dynamic*/ v_multiply =
     (/*error:TOP_LEVEL_UNSUPPORTED*/ new A(). /*@target=A::f*/ f *= 1);
-var /*@topType=dynamic*/ v_prefix_pp = (/*@target=num::+*/ ++new A()
-    . /*error:TOP_LEVEL_INSTANCE_GETTER*/ /*@target=A::f*/ f);
-var /*@topType=dynamic*/ v_prefix_mm = (/*@target=num::-*/ --new A()
-    . /*error:TOP_LEVEL_INSTANCE_GETTER*/ /*@target=A::f*/ f);
+var /*@topType=dynamic*/ v_prefix_pp =
+    (++new A(). /*error:TOP_LEVEL_INSTANCE_GETTER*/ /*@target=A::f*/ f);
+var /*@topType=dynamic*/ v_prefix_mm =
+    (--new A(). /*error:TOP_LEVEL_INSTANCE_GETTER*/ /*@target=A::f*/ f);
 var /*@topType=dynamic*/ v_postfix_pp =
     (new A(). /*error:TOP_LEVEL_INSTANCE_GETTER*/ /*@target=A::f*/ f++);
 var /*@topType=dynamic*/ v_postfix_mm =
     (new A(). /*error:TOP_LEVEL_INSTANCE_GETTER*/ /*@target=A::f*/ f--);
 
 main() {}