Fix several little buglets with stepping

tests/standalone/debugger/step_in_equals_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";
-
-class MyClass {
-  operator ==(other) {
-    print(other);
-    return true;  // Breakpoint #3.
-  }
-}
-
-main(List<String> arguments) {
-  if (RunScript(testScript, arguments)) return;
-  var a = new MyClass();
-  var b = null;
-  var x = a == b;  // Breakpoint #1.
-  print(x);
-  b = 123;
-  a == b;  // Breakpoint #2.
-  print("after 2");
-  if (a == null) {  // Breakpoint #4.
-    throw "unreachable";
-  }
-  print("after 4");
-  if (null == a) {
-    throw "unreachable";
-  }
-  print("ok");
-}
-
-var testScript = [
-  MatchFrames(["main"]),
-  SetBreakpoint(18),
-  SetBreakpoint(21),
-  SetBreakpoint(10),
-  SetBreakpoint(23),
-  Resume(),
-  MatchFrames(["main"]),  // At breakpoint #1.
-  StepInto(),
-  MatchFrames(["main"]),  // Don't step into == method because of null.
-  Resume(),
-  MatchFrames(["main"]), // At breakpoint #2.
-  StepInto(),

[Florian Schneider, 2014/01/20 11:36:28]

Remove one StepInto. The bug that requires 2 steps seems to be fixed by now.

-  StepInto(),
-  MatchFrames(["MyClass.==", "main"]),  // At MyClass.== entry.
-  Resume(),
-  MatchFrames(["MyClass.==", "main"]),  // At breakpoint #3.
-  Resume(),
-  MatchFrames(["main"]), // At breakpoint #4.
-  StepInto(),
-  MatchFrames(["main"]), // After breakpoint #4.
-  Step(),
-  MatchFrames(["main"]), // At null == a.
-  StepInto(),
-  MatchFrames(["main"]), // After null == a.
-  Resume()
-];