Rationalize Dart mojo and sky package structure

sky/framework/debug/shake-to-reload.sky

-<script>
-import 'dart:sky';
-import 'package:mojom/sensors/sensors.mojom.dart';
-import 'package:sky/framework/shell.dart' as shell;
-
-// TODO(abarth): We should factor this out into a kinematics library.
-class _ShakeDetector implements SensorListener {
-  _ShakeDetector() {
-    SensorServiceProxy sensorService = new SensorServiceProxy.unbound();
-    shell.requestService("mojo:sensors", sensorService);
-
-    _stub = new SensorListenerStub.unbound()..impl = this;
-    sensorService.ptr.addListener(SensorType_ACCELEROMETER, _stub);
-  }
-
-  void onAccuracyChanged(int accuracy) {
-  }
-
-  void onSensorChanged(SensorData data) {
-    double value = data.values[0] + data.values[1] + data.values[2];
-    if (isShaking && value < 15.0)
-      didCompleteShake();
-    else if (value > 40.0)
-      isShaking = true;
-  }
-
-  void didCompleteShake() {
-    window.location.assign(document.URL);
-    _stub.close();
-  }
-
-  bool isShaking = false;
-  SensorListenerStub _stub;
-}
-
-_ShakeDetector _detector;
-
-void _init(_) {
-  _detector = new _ShakeDetector();
-}
-</script>