Lift nearly all top-level type inference restrictions.

pkg/front_end/testcases/inference_new/infer_assign_to_ref.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;
 }
 
 A a = new A();
 var /*@topType=dynamic*/ b =
     (/*error:TOP_LEVEL_UNSUPPORTED*/ a. /*@target=A::f*/ f = 1);
 var /*@topType=int*/ c = 0;
-var /*@topType=dynamic*/ d = (/*error:TOP_LEVEL_UNSUPPORTED*/ c = 1);
+var /*@topType=int*/ d = (c = 1);
 
 main() {
   a;
   b;
   c;
   d;
 }