Debugger: correctly break in default constructor.

R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/1423673003

Cr-Commit-Position: refs/heads/master@{#31665}

src/full-codegen/full-codegen.cc

@@ -560,7 +560,10 @@ void FullCodeGenerator::SetFunctionPosition(FunctionLiteral* fun) {
 
 
 void FullCodeGenerator::SetReturnPosition(FunctionLiteral* fun) {
-  RecordStatementPosition(masm_, fun->end_position() - 1);
+  // For default constructors, start position equals end position, and there
+  // is no source code besides the class literal.
+  int pos = std::max(fun->start_position(), fun->end_position() - 1);
+  RecordStatementPosition(masm_, pos);
   if (info_->is_debug()) {
     // Always emit a debug break slot before a return.
     DebugCodegen::GenerateSlot(masm_, RelocInfo::DEBUG_BREAK_SLOT_AT_RETURN);

test/mjsunit/es6/debug-break-default-constructor.js

+// Copyright 2015 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: --expose-debug-as debug
+
+"use strict";
+
+var Debug = debug.Debug;
+var exception = null;
+var super_called = false;
+var step_count = 0;
+
+function listener(event, execState, eventData, data) {
+  if (event != Debug.DebugEvent.Break) return;
+  try {
+    execState.prepareStep(Debug.StepAction.StepInto);
+    var s = execState.frame().sourceLineText();
+    step_count++;
+    assertTrue(s.indexOf('// ' + step_count + '.') >= 0);
+  } catch (e) {
+    exception = e;
+  }
+}
+
+class Base {
+  constructor() {  // 2.
+    var x = 1;     // 3.
+  }                // 4.
+}
+
+class Derived extends Base {}  // 1. // 5.
+
+Debug.setListener(listener);
+var bp = Debug.setBreakPoint(Derived, 0);
+
+new Derived();
+
+Debug.setListener(null);  // 6.
+
+assertNull(exception);
+assertEquals(6, step_count);