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

test/mjsunit/harmony/async-debug-step-abort-at-break.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 --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( // B3 StepOut
+    function(res, rej) {
+      late_resolve = res;
+    }
+  );
+}
+
+async function f() {
+  var a = 1;
+  debugger;          // B0 StepNext
+  a +=               // B1 StepNext
+       await         // B4 StepNext
+             g();    // B2 StepIn
+  return a;
+}
+
+f();
+
+// Starting a new step action at an intermediate break point
+// means that we will abort the current async step.
+debugger;            // B5 StepNext
+
+late_resolve(3);     // B6 Continue
+
+%RunMicrotasks();
+
+assertEquals(7, step_count);