Implement override-based type inference for instance methods.

pkg/front_end/testcases/inference/infer_from_complex_expressions_if_outer_most_value_is_precise.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 x;
-  B operator +(other) => null;
+  B operator +(/*@topType=dynamic*/ other) => null;
 }
 
 class B extends A {
   B(ignore);
 }
 
 var /*@topType=A*/ a = new A();
 // Note: it doesn't matter that some of these refer to 'x'.
 var /*@topType=B*/ b = new B(/*error:UNDEFINED_IDENTIFIER*/ x); // allocations
 var /*@topType=List<dynamic>*/ c1 = /*@typeArgs=dynamic*/ [
@@ skipping 32 matching lines @@
   g = /*error:INVALID_ASSIGNMENT*/ false;
   h = /*error:INVALID_ASSIGNMENT*/ false;
   h = new B('b');
   i = false;
   j = new B('b');
   j = /*error:INVALID_ASSIGNMENT*/ false;
   j = /*error:INVALID_ASSIGNMENT*/ /*@typeArgs=dynamic*/ [];
 }
 
 main() {}