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;

[hausner, 2013/07/02 21:48:14]

If this still gives you trouble to repro the bug we fixed yesterday, try to put
the print statements into a separate function.

void foo() {
  print('foo1');
  print('foo2');
  print('foo3');
}

main() {
  if (RunScript(testScript)) return;
  foo();
}

[devoncarew, 2013/07/02 22:12:42]

Ah, good idea. I was able to repro the issue having everything in one main()
method, but moving the test code into a separate method more closely matches the
repro case I had yesterday.

So, the current test does fail w/ yesterday's codebase, and passes w/ the fix to
debugger.cc.

+  
+  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(10),
+  Resume(),
+  MatchFrames(["main"]),
+  Step(),
+  MatchFrames(["main"]),
+  Resume(),
+];