Added a test to cover debugging step over.

tests/standalone/debugger/stepping_test.dart

+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import "debug_lib.dart";
+
+main() {
+  if (RunScript(testScript)) return;
+  
+  bar();
+}
+
+void bar() {
+  print('foo1');
+  print('foo2');
+  print('foo3');
+}
+
+/**
+ * Set a breakpoint, resume to that breakpoint, step once, and verify that the
+ * step worked.
+ */
+var testScript = [
+  MatchFrames(["main"]),
+  SetBreakpoint(15),
+  Resume(),
+  MatchFrames(["bar"]),
+  Step(),
+  MatchFrames(["bar"]),
+  Resume(),
+];