[debugger] add precise mode for code coverage.

test/mjsunit/code-coverage-precise.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 precise code coverage.
+
+function GetCoverage(source) {
+  var scripts = %DebugGetLoadedScripts();
+  for (var script of scripts) {
+    if (script.source == source) {
+      var coverage = %DebugCollectCoverage();
+      for (var data of coverage) {
+        if (data.script_id == script.id) return data.entries;
+      }
+    }
+  }
+  return undefined;
+}
+
+function ApplyCoverageToSource(source, coverage) {
+  var result = "";
+  var cursor = 0;
+  for (var entry of coverage) {
+    var chunk = source.substring(cursor, entry.end_position);
+    cursor = entry.end_position;
+    result += `[${chunk}[${entry.count}]]`;
+  }
+  return result;
+}
+
+function TestCoverage(name, source, expectation) {
+  source = source.trim();
+  eval(source);
+  var coverage = GetCoverage(source);
+  if (expectation === undefined) {
+    assertEquals(undefined, coverage);
+  } else {
+    expectation = expectation.trim();
+    var result = ApplyCoverageToSource(source, coverage);
+    print(result);
+    assertEquals(expectation, result, name + " failed");
+  }
+}
+
+
+// Without precise coverage enabled, we lose coverage data to the GC.
+TestCoverage(
+"call an IIFE",
+`
+(function f() {})();
+`,
+undefined  // The IIFE has been garbage-collected.
+);
+
+TestCoverage(
+"call locally allocated function",
+`
+for (var i = 0; i < 10; i++) {
+  let f = () => 1;
+  i += f();
+}
+`,
+undefined
+);
+
+// This does not happen with precise coverage enabled.
+%DebugTogglePreciseCoverage(true);
+
+TestCoverage(
+"call an IIFE",
+`
+(function f() {})();
+`,
+`
+[(function f() {})();[1]]
+`
+);
+
+TestCoverage(
+"call locally allocated function",
+`
+for (var i = 0; i < 10; i++) {
+  let f = () => 1;
+  i += f();
+}
+`,
+`
+[for (var i = 0; i < 10; i++) {
+  let f = [1]][() => 1[5]][;
+  i += f();
+}[1]]
+`
+);
+
+%DebugTogglePreciseCoverage(false);