[debug] introduce precise binary code coverage.

test/mjsunit/code-coverage-ad-hoc.js

 // Copyright 2017 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Flags: --allow-natives-syntax --no-always-opt
 
 // Test code coverage without explicitly activating it upfront.
 
 function GetCoverage(source) {
   for (var script of %DebugCollectCoverage()) {
@@ skipping 12 matching lines @@
 }
 
 TestCoverage(
 "call simple function twice",
 `
 function f() {}
 f();
 f();
 `,
 [{"start":0,"end":25,"count":1},
- {"start":0,"end":15,"count":2}]
+ {"start":0,"end":15,"count":1}]
 );
 
 TestCoverage(
 "call arrow function twice",
 `
 var f = () => 1;
 f();
 f();
 `,
 [{"start":0,"end":26,"count":1},
- {"start":8,"end":15,"count":2}]
+ {"start":8,"end":15,"count":1}]
 );
 
 TestCoverage(
 "call nested function",
 `
 function f() {
   function g() {}
   g();
   g();
 }
 f();
 f();
 `,
 [{"start":0,"end":58,"count":1},
- {"start":0,"end":48,"count":2},
- {"start":17,"end":32,"count":4}]
+ {"start":0,"end":48,"count":1},
+ {"start":17,"end":32,"count":1}]
 );
 
 TestCoverage(
 "call recursive function",
 `
 function fib(x) {
   if (x < 2) return 1;
   return fib(x-1) + fib(x-2);
 }
 fib(5);
 `,
 [{"start":0,"end":80,"count":1},
- {"start":0,"end":72,"count":15}]
+ {"start":0,"end":72,"count":1}]
 );