Add version validation for LookupMap, also add unittest directly in LookupMap (take 2)

pkg/lookup_map/lib/lookup_map.dart

Index: pkg/lookup_map/lib/lookup_map.dart
diff --git a/pkg/lookup_map/lib/lookup_map.dart b/pkg/lookup_map/lib/lookup_map.dart
index 609f854288d10088c23cfc397cbda82b4ea84aeb..0c03094e680e8590419a0c537798c054e9aef8c5 100644
--- a/pkg/lookup_map/lib/lookup_map.dart
+++ b/pkg/lookup_map/lib/lookup_map.dart
@@ -91,3 +91,10 @@ class LookupMap<K, V> {
 /// An expando that stores a flatten version of a [LookupMap], this is
 /// computed and stored the first time the map is accessed.
 final _flatMap = new Expando('_flat_map');
+
+/// Internal constant that matches the version in the pubspec. This is used by
+/// dart2js to ensure that optimizations are only enabled on known versions of
+/// this code.
+// Note: this needs to be kept in sync with the pubspec, otherwise
+// test/version_check_test would fail.
+final _version = '0.0.1';