Support an @OnPlatform annotation.

lib/src/utils.dart

Index: lib/src/utils.dart
diff --git a/lib/src/utils.dart b/lib/src/utils.dart
index 3b8111d0f379e9a1171f397c54c42e0984c937e8..f236814aa7a902cfe79a74a67f269a4c4d81cf72 100644
--- a/lib/src/utils.dart
+++ b/lib/src/utils.dart
@@ -51,6 +51,23 @@ final OperatingSystem currentOSGuess = (() {
   return OperatingSystem.linux;
 })();
 
+/// A pair of values.
+class Pair<E, F> {
+  E first;
+  F last;
+
+  Pair(this.first, this.last);
+
+  String toString() => '($first, $last)';
+
+  bool operator==(other) {
+    if (other is! Pair) return false;
+    return other.first == first && other.last == last;
+  }
+
+  int get hashCode => first.hashCode ^ last.hashCode;
+}
+
 /// Get a string description of an exception.
 ///
 /// Many exceptions include the exception class name at the beginning of their