[debugger] add test case for stepping into string template.

test/mjsunit/es6/debug-stepin-string-template.js

Index: test/mjsunit/es6/debug-stepin-string-template.js
diff --git a/test/mjsunit/es6/debug-stepin-string-template.js b/test/mjsunit/es6/debug-stepin-string-template.js
new file mode 100644
index 0000000000000000000000000000000000000000..86a7bb9b5d895de0d753e6b121983894df32895a
--- /dev/null
+++ b/test/mjsunit/es6/debug-stepin-string-template.js
@@ -0,0 +1,60 @@
+// 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
+
+Debug = debug.Debug
+
+var exception = null;
+var log = [];
+
+function listener(event, exec_state, event_data, data) {
+  if (event != Debug.DebugEvent.Break) return;
+  try {
+    print(event_data.sourceLineText());
+    var entry = "";
+    for (var i = 0; i < exec_state.frameCount(); i++) {
+      entry += exec_state.frame(i).sourceLineText().substr(-1);
+      entry += exec_state.frame(i).sourceColumn();
+    }
+    log.push(entry);
+    exec_state.prepareStep(Debug.StepAction.StepIn, 1);
+  } catch (e) {
+    exception = e;
+  }
+};
+
+function u(x) {
+  return x.toUpperCase();                         // d
+}                                                 // e
+
+var n = 3;
+
+var o = {
+  toString: function() {
+    return "D";                                   // f
+  }                                               // g
+}
+
+
+
+Debug.setListener(listener);
+debugger;                                         // a
+var s = `1 ${u("a")} 2 ${u("b")} 3 ${n} 4 ${o}`;  // b
+Debug.setListener(null);                          // c
+
+assertNull(exception);
+
+assertEquals([
+  "a0",
+  "b0",
+  "d2b13",
+  "e0b13",
+  "b25",
+  "d2b25",
+  "e0b25",
+  "f4b44",
+  "g2b44",
+  "c0"
+], log);