add dart examples for chapter 1 and 2 (#209)

* add dart example for chapter 1

* add dart example for chapter 2

02_selection_sort/dart/selection_sort.dart

@@ -0,0 +1,29 @@
+main() {
+  print(selectionSort([5, 3, 6, 2, 10]));
+}
+
+List<int> selectionSort(List<int> arr) {
+  final List<int> newArr = List.empty(growable: true);
+  final arrLength = arr.length;
+
+  for (int i = 0; i < arrLength; i++) {
+    final smallest = findSmallest(arr);
+    newArr.add(arr.removeAt(smallest));
+  }
+
+  return newArr;
+}
+
+int findSmallest(List<int> arr) {
+  int smallest = arr[0];
+  int smallestIndex = 0;
+
+  for (int i = 1; i < arr.length; i++) {
+    if (arr[i] < smallest) {
+      smallest = arr[i];
+      smallestIndex = i;
+    }
+  }
+
+  return smallestIndex;
+}