[debug] implement intuitive semantics for stepping over await call.

test/mjsunit/harmony/async-debug-step-nested.js

Index: test/mjsunit/harmony/async-debug-step-nested.js
diff --git a/test/mjsunit/harmony/async-debug-step-nested.js b/test/mjsunit/harmony/async-debug-step-nested.js
new file mode 100644
index 0000000000000000000000000000000000000000..adf7a51432579cd87badfec4ecaf83faf717d19c
--- /dev/null
+++ b/test/mjsunit/harmony/async-debug-step-nested.js
@@ -0,0 +1,58 @@
+// 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 --allow-natives-syntax --harmony-async-await
+
+var Debug = debug.Debug;
+var step_count = 0;
+
+function listener(event, execState, eventData, data) {
+  if (event != Debug.DebugEvent.Break) return;
+  try {
+    var line = execState.frame(0).sourceLineText();
+    print(line);
+    var [match, expected_count, step] = /\/\/ B(\d) (\w+)$/.exec(line);
+    assertEquals(step_count++, parseInt(expected_count));
+    if (step != "Continue") execState.prepareStep(Debug.StepAction[step]);
+  } catch (e) {
+    print(e, e.stack);
+    quit(1);
+  }
+}
+
+Debug.setListener(listener);
+
+var late_resolve;
+
+function g() {
+  return new Promise( // B4 StepOut
+    function(res, rej) {
+      late_resolve = res;
+    }
+  );
+}
+
+async function f1() {
+  var a = 1;
+  debugger;          // B0 StepNext
+  a +=               // B1 StepNext
+       await         // B6 StepNext
+             f2();   // B2 StepIn
+  return a;          // B7 StepNext
+}                    // B8 Continue
+
+async function f2() {
+  var b =
+          await      // B5 StepOut
+                g(); // B3 StepIn
+  return b;
+}
+
+f1();
+
+late_resolve(3);
+
+%RunMicrotasks();
+
+assertEquals(9, step_count);