Fix several little buglets with stepping

tests/standalone/debugger/break_at_equals_test.dart

+// Copyright (c) 2014, 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;
+  a == b;  // Breakpoint #1.
+  print("after 1");
+  b = 123;
+  a == b;  // Breakpoint #2.
+  print("after 2");
+  a == null;  // Breakpoint #4.
+  print("after 4");
+  if (null == a) {
+    throw "unreachable";
+  }
+  print("ok");
+}
+
+// Checks that debugger can stop at calls to == even if one
+// of the operands is null.
+
+var testScript = [
+  MatchFrames(["main"]),
+  MatchLine(15),
+  SetBreakpoint(18),
+  SetBreakpoint(21),
+  SetBreakpoint(10),
+  SetBreakpoint(23),
+  Resume(),
+  MatchFrames(["main"]),
+  MatchLine(18),  // At breakpoint #1.
+  Resume(),
+  MatchLine(21),  // At breakpoint #2; didn't hit breakpoint in MyClass.==.
+  Resume(),
+  MatchFrames(["MyClass.==", "main"]),
+  MatchLine(10),  // At breakpoint #3 in MyClass.==.
+  Resume(),
+  MatchLine(23),  // At breakpoint #4.
+  Resume()

[Florian Schneider, 2014/01/17 10:18:01]

Why doesn't this new test does test single stepping at all?

I think stepping into/over == operations should be covered.

+];