Update sky HACKING.md instructions to use mojo_shell.py.

sky/HACKING.md

 Hacking on Sky
 ==============
 
 Building
 --------
 
 * Follow the setup & build instructions for [Mojo](https://github.com/domokit/mojo)
 
 The build directory will be `out/Debug` for Linux debug builds, and
 `out/Release` for Linux release builds. For Android builds, prefix
 `android_`, as in, `android_Debug`.
 
 For Sky on iOS, you can try following these [experimental instructions](https://docs.google.com/document/d/1qm8Vvyz8Mngw6EsSg4FELQ7FOegjBJ5Itg9OBJGN5JM/edit#heading=h.ikz9bdwswdct).
 
 Running applications
 --------------------
 
 To run an application on your device, run:
 
-* ``./mojo/tools/mojodb start out/android_Debug [url]``
+* `mojo/devtools/common/mojo_shell --sky [url] --android`
 
-`mojodb` has numerous commands, visible via `mojodb help`.  Common ones include:
-* `mojodb start` BUILD_DIR [url]
-* `mojodb load` [url]
-* `mojodb stop`
-* `mojodb start_tracing` # Starts recoding a performance trace (use stop_tracing to stop)
-* `mojodb print_crash` # Symbolicate the most recent crash from android.
-
-Once `mojodb start` is issued, all subsequent commands will be sent to
-the running mojo_shell instance (even on an attached android device).
-`mojodb start` reads gn args from the passed build directory to
-determine whether its using android, for example.
+When the shell is running, `mojo/devtools/common/debugger` allows you to
+collect traces, symbolize stack crashes and attach gdb if needed. Refer to the
+[documentation](https://github.com/domokit/mojo#debugging-tracing-profiling)
+for the details.
 
 Running tests
 -------------
 
 Tests are only supported on Linux currently.
 
-* ``./sky/tools/test_sky --debug``
+* ``sky/tools/test_sky --debug``
   * This runs the tests against ``//out/Debug``. If you want to run against
     ``//out/Release``, omit the ``--debug`` flag.
 
 Running tests manually
 ----------------------
 
 Running tests manually lets you more quickly iterate during
 development; rather than having to compile and rerun all the tests,
 then trawl through the build output to find the current results, you
 can just run the test you're working on and reload it over and over,
@@ skipping 13 matching lines @@
 See [``sky/tests/raw/color_bounds.dart``](tests/raw/color_bounds.dart)
 for an example.
 
 Debugging Sky
 -------------
 
 This document aims to explain how to debug Sky itself.
 
 ### C++
 
-Launch a debug Sky build on Linux as follows (where `app.dart` is the
-test you are running and trying to debug):
+Launch a debug Sky build on Android as usual:
 
-```bash
-mojodb start --gdb out/Debug app.dart
-mojodb gdb_attach
+```
+mojo/devtools/common/mojo_shell --sky [url] --android`
 ```
 
-Once gdb has loaded, hit `c` to start the app. The linux simulator
-will load (slowly), after spawning many threads. When your app

[Hixie, 2015/07/16 15:40:39]

Is it no longer slow? That's great news if so!

-crashes, it will pause in the debugger. At that point, regular gdb
-commands will work: `n` to step over the current statement, `s` to
-step into the current statement, `f` to step out of the current block,
-`c` to continue until the next breakpoint or exception.
+and use the debugger to attach gdb:
+```
+mojo/devtools/common/debugger gdb attach
+```
+
+Once gdb has loaded, hit `c` to continue the execution. When your app crashes,
+it will pause in the debugger. At that point, regular gdb commands will work:
+`n` to step over the current statement, `s` to step into the current statement,
+`f` to step out of the current block, `c` to continue until the next breakpoint
+or exception.
 
 ### Dart
 
 Use Observatory.