Add Google Maps Place Tracker sample app to flutter/samples. (#20)

Sample app that displays places on a map. Add/edit places. Interact with map. Iterations to follow.

place_tracker/lib/place.dart

@@ -0,0 +1,46 @@
+import 'package:flutter/material.dart';
+import 'package:google_maps_flutter/google_maps_flutter.dart';
+
+enum PlaceCategory {
+  favorite,
+  visited,
+  wantToGo,
+}
+
+class Place {
+  const Place({
+    @required this.latLng,
+    @required this.name,
+    @required this.category,
+    this.description,
+    this.starRating = 0,
+  }) : assert(latLng != null),
+       assert(name != null),
+       assert(category != null),
+       assert(starRating != null && starRating >= 0 && starRating <= 5);
+
+  final LatLng latLng;
+  final String name;
+  final PlaceCategory category;
+  final String description;
+  final int starRating;
+
+  double get latitude => latLng.latitude;
+  double get longitude => latLng.longitude;
+
+  Place copyWith({
+    LatLng latLng,
+    String name,
+    PlaceCategory category,
+    String description,
+    int starRating,
+  }) {
+    return Place(
+        latLng: latLng ?? this.latLng,
+        name: name ?? this.name,
+        category: category ?? this.category,
+        description: description ?? this.description,
+        starRating: starRating ?? this.starRating,
+    );
+  }
+}