[debugger] fix switch block source positions.

test/debugger/regress/regress-6085.js

+// Copyright 2017 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.
+
+function* serialize() {
+  debugger;
+  switch (0) {
+    case 0:
+      let x = 1;
+      return x;  // Check scopes
+  }
+}
+
+let exception = null;
+let step_count = 0;
+let scopes_checked = false;
+
+function listener(event, exec_state, event_data, data) {
+  if (event != Debug.DebugEvent.Break) return;
+  try {
+    if (exec_state.frame().sourceLineText().includes("Check scopes")) {
+      let expected = [ debug.ScopeType.Block,
+                       debug.ScopeType.Local,
+                       debug.ScopeType.Script,
+                       debug.ScopeType.Global ];
+      for (let i = 0; i < exec_state.frame().scopeCount(); i++) {
+        assertEquals(expected[i], exec_state.frame().scope(i).scopeType());
+      }
+      scopes_checked = true;
+    }
+    if (step_count++ < 3) exec_state.prepareStep(Debug.StepAction.StepNext);
+  } catch (e) {
+    exception = e;
+    print(e, e.stack);
+  }
+}
+
+
+
+let Debug = debug.Debug;
+Debug.setListener(listener);
+
+let gen = serialize();
+gen.next();
+
+Debug.setListener(null);
+assertNull(exception);
+assertEquals(4, step_count);
+assertTrue(scopes_checked);