Add 'breakpoint' similar to 'debugger' from JavaScript.

dart/sdk/lib/core/breakpoint.dart

Index: dart/sdk/lib/core/breakpoint.dart
diff --git a/dart/tests/language/deferred_constraints_lib.dart b/dart/sdk/lib/core/breakpoint.dart
similarity index 61%
copy from dart/tests/language/deferred_constraints_lib.dart
copy to dart/sdk/lib/core/breakpoint.dart
index 9f4d207ce6f24744420d888a98714677f5665668..0dfc4e8c90d5085668325aac83d1e44520044b6c 100644
--- a/dart/tests/language/deferred_constraints_lib.dart
+++ b/dart/sdk/lib/core/breakpoint.dart
@@ -2,14 +2,8 @@
 // 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.
 
-class C {
-  static int staticMethod() => 42;
-}
+part of dart.core;
 
-class G<T> {}
-
-class Const {
-  const Const();
-}
-
-const constantInstance = const Const();
+/// Pause execution during debugging, a breakpoint.

[Lasse Reichstein Nielsen, 2014/03/20 08:25:39]

What does it means to be "during debugging". I'm always debugging my code, but I
usually do it by inserting print statements.

How about more text to elaborate it:

Returns `null` when resuming after the pause.

If an external debugger is attached to the program, reading this getter will
trigger that debugger and allow it to interrupt the program at this point.
If no debugger is present, reading `breakpoint` does nothing, it just returns
`null` normally.

+/// Has no effect when not debugging.
+external void get breakpoint;

[Lasse Reichstein Nielsen, 2014/03/20 08:25:39]

This is regrettably a breaking change (no pun intended).

The lower-case name can conflict with instance variables in user classes, and
due to Dart's scoping rules, this declaration will override unqualified
references to that variable in subclasses.

[Lasse Reichstein Nielsen, 2014/03/20 08:25:39]

"external void get" is not allowed by the spec. The return type of top-level
external getters must match "type?", which does not include "void".
(I think it is a mistake that top-level non-external getters do allow void -
class getter members don't either).

[kevmoo, 2014/03/20 16:34:46]

DBC: Can we make this a method?

break() or setBreakPoint()

This has a side effect. Let's not copy Javascript unless it makes sense.

[ahe, 2014/03/20 16:50:47]

Breakpoint is one word according to DevTools, and the spell checker in Chrome.

I don't think setBreakpoint would be a good fit. This *is* a break point. A
method named "setBreakpoint" would be found in a  debugger and its arguments
would be a location.
It doesn't really have a side-effect in terms of normal language semantics. But
Florian provided a really good argument for having it take one argument. So I'm
all for making it a method.