Add hotkeys to Observatory debugger.

Add hotkeys to Observatory debugger.

Add 'cls' command to clear the console.

From 'help hotkeys':

List of hotkeys:

[TAB]        - complete a command
[Up Arrow]   - history previous
[Down Arrow] - history next

[Page Up]    - move up one frame
[Page Down]  - move down one frame

[F7]         - continue execution of the current isolate
[Ctrl ;]     - pause execution of the current isolate

[F8]         - toggle breakpoint at current location
[F9]         - next
[F10]        - step


BUG=24010
R=johnmccutchan@google.com

Committed: https://github.com/dart-lang/sdk/commit/97a4ae8f9496b6740cff67b1664258b51c0a7f68

[turnidge, 2015-08-07 02:16:33]

turnidge@google.com changed reviewers:
+ johnmccutchan@google.com

[Cutch, 2015-08-07 13:41:08]

On 2015/08/07 02:16:34, turnidge wrote:

LGTM but we should make sure that these keyboard shortcuts work on Mac, Linux,
and Windows.

[turnidge, 2015-08-07 17:25:10]

On 2015/08/07 13:41:08, Cutch wrote:
> On 2015/08/07 02:16:34, turnidge wrote:
> 
> LGTM but we should make sure that these keyboard shortcuts work on Mac, Linux,
> and Windows.

I have change the keybindings to be slightly more platform neutral... take a
look?

[turnidge, 2015-08-07 17:38:44]

Committed patchset #5 (id:80001) manually as
97a4ae8f9496b6740cff67b1664258b51c0a7f68 (presubmit successful).