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

* add dart example for chapter 1

* add dart example for chapter 2

01_introduction_to_algorithms/dart/binary_search.dart

@@ -0,0 +1,25 @@
+void main() {
+  final myList = [1, 3, 5, 7, 9];
+  print(binarySearch(myList, 3));
+  print(binarySearch(myList, -1));
+}
+
+int? binarySearch(List<int> list, int item) {
+  int low = 0;
+  int high = list.length - 1;
+
+  while (low <= high) {
+    final mid = (low + high) ~/ 2;
+    final guess = list[mid];
+
+    if (guess == item) {
+      return mid;
+    }
+
+    if (guess > item) {
+      high = mid - 1;
+    } else {
+      low = mid + 1;
+    }
+  }
+}