[ignition, debugger] correctly set position for return with elided bytecode.

test/mjsunit/ignition/elided-instruction.js

+// Copyright 2016 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 --ignition --nostress-opt --noalways-opt

[rmcilroy, 2016/03/14 14:54:55]

I guess this will set clusterfuzz on ignition, but given we are talking about
doing that anyway explicitly this should be ok.

+
+Debug = debug.Debug
+
+var exception = null;
+var break_count = 0;
+
+function listener(event, exec_state, event_data, data) {
+  if (event != Debug.DebugEvent.Break) return;
+  try {
+    print(event_data.sourceLineText());
+    var column = event_data.sourceColumn();
+    assertTrue(event_data.sourceLineText().indexOf(
+        `Break ${break_count++}. ${column}.`) > 0);
+    exec_state.prepareStep(Debug.StepAction.StepIn);
+  } catch (e) {
+    print(e + e.stack);
+    exception = e;
+  }
+};
+
+function f() {
+  var a = 1;                     // Break 2. 10.
+  // This return statement emits no bytecode instruction for the evaluation of
+  // the to-be-returned expression. Therefore we cannot set a break location
+  // before the statement and a second break location immediately before
+  // returning to the caller.
+  return a;
+}                                // Break 3. 0.
+
+Debug.setListener(listener);
+debugger;                        // Break 0. 0.
+f();                             // Break 1. 0.
+Debug.setListener(null);         // Break 4. 0.
+
+assertNull(exception);
+assertEquals(5, break_count);