Use counter for legacy const.

We only report the usages when full parse is happening, i.e. only when the function that declares a legacy const is compiled. This is an approximation that is easy to implement, but still should reflect the real-world usage.

BUG=v8:3942
LOG=N

Review URL: https://codereview.chromium.org/1041863002

Cr-Commit-Position: refs/heads/master@{#27526}

src/parser.cc

@@ -2272,6 +2272,7 @@ Block* Parser::ParseVariableDeclarations(
     if (is_sloppy(language_mode())) {
       mode = CONST_LEGACY;
       init_op = Token::INIT_CONST_LEGACY;
+      ++use_counts_[v8::Isolate::kLegacyConst];
     } else {
       DCHECK(var_context != kStatement);
       mode = CONST;

test/cctest/test-parsing.cc

@@ -3441,6 +3441,25 @@ TEST(UseAsmUseCount) {
 }
 
 
+TEST(UseConstLegacyCount) {
+  i::Isolate* isolate = CcTest::i_isolate();
+  i::HandleScope scope(isolate);
+  LocalContext env;
+  int use_counts[v8::Isolate::kUseCounterFeatureCount] = {};
+  global_use_counts = use_counts;
+  CcTest::isolate()->SetUseCounterCallback(MockUseCounterCallback);
+  CompileRun(
+      "const x = 1;\n"
+      "var foo = 1;\n"
+      "const y = 1;\n"
+      "function bar() {\n"
+      "    const z = 1; var baz = 1;\n"
+      "    function q() { const k = 42; }\n"
+      "}");
+  CHECK_EQ(4, use_counts[v8::Isolate::kLegacyConst]);
+}
+
+
 TEST(ErrorsArrowFunctions) {
   // Tests that parser and preparser generate the same kind of errors
   // on invalid arrow function syntax.