Allow the shell runner to run outside of Mojo checkout.

mojo/devtools/common/mojo_shell

Index: mojo/devtools/common/mojo_shell
diff --git a/mojo/devtools/common/mojo_shell b/mojo/devtools/common/mojo_shell
index 3b4d77bef4eecdef657dab5154f4d34a7e76e9f4..edb101221517e60b18847a6bbcc49d016ce3886b 100755
--- a/mojo/devtools/common/mojo_shell
+++ b/mojo/devtools/common/mojo_shell
@@ -74,30 +74,49 @@ def main():
                       'for off-screen rendering.')
 
   launcher_args, args = parser.parse_known_args()
-  paths, error_msg = default_paths.infer_default_paths(launcher_args.android,
-                                                       launcher_args.debug,
-                                                       launcher_args.target_cpu)
-  if not paths:
-    print error_msg
-    return -1
+  mojo_paths, _ = default_paths.infer_default_paths(launcher_args.android,
+                                                    launcher_args.debug,
+                                                    launcher_args.target_cpu)
+  if mojo_paths:
+    adb_path = mojo_paths['adb']
+    shell_binary_path = mojo_paths['shell']
+    local_origin_path = mojo_paths['build']
+    if launcher_args.verbose:
+      print 'Running within a Mojo checkout:'
+      print ' - using the locally built shell at ' + shell_binary_path
+      print ' - using the default origin of ' + local_origin_path
+  else:
+    if launcher_args.android:
+      adb_path = 'adb'
+      shell_binary_path = None
+      local_origin_path = '.'
+      if launcher_args.verbose:
+        print 'Running outside a Mojo checkout:'
+        print ' - using the shell already installed on the device'
+        print ' - using the current working directory as default origin'
+    else:
+      print 'Running outside a Mojo checkout is not supported on Linux yet.'
+      return 1
 
   if launcher_args.android:
     verbose_pipe = sys.stdout if launcher_args.verbose else None
 
-    shell = AndroidShell(paths['adb'], launcher_args.target_device,
+    shell = AndroidShell(adb_path, launcher_args.target_device,
                          logcat_tags=launcher_args.logcat_tags,
                          verbose_pipe=verbose_pipe)
     device_status, error = shell.CheckDevice()
     if not device_status:
       print 'Device check failed: ' + error
       return 1
-    shell.InstallApk(paths['shell'])
+    if shell_binary_path:
+      shell.InstallApk(shell_binary_path)
 
     args = shell_arguments.RewriteMapOriginParameters(shell, args)
     if not launcher_args.origin:
-      args.extend(shell_arguments.ConfigureLocalOrigin(shell, paths['build']))
+      args.extend(shell_arguments.ConfigureLocalOrigin(shell,
+                                                       local_origin_path))
   else:
-    shell = LinuxShell(paths['shell'])
+    shell = LinuxShell(shell_binary_path)
     if launcher_args.use_osmesa:
       args.append('--args-for=mojo:native_viewport_service --use-osmesa')
 
@@ -110,8 +129,12 @@ def main():
     args.extend(shell_arguments.ConfigureDebugger(shell))
 
   if launcher_args.sky:
-    args.extend(shell_arguments.ConfigureSky(shell, paths['root'],
-                                             paths['sky_packages'],
+    if not mojo_paths:
+      print 'Running with --sky is not supported outside of the Mojo checkout.'
+      # See https://github.com/domokit/devtools/issues/27.
+      return 1
+    args.extend(shell_arguments.ConfigureSky(shell, mojo_paths['root'],
+                                             mojo_paths['sky_packages'],
                                              launcher_args.sky))
 
   if launcher_args.verbose: