Fix typo in lookup map, use pub_semver to check version constraints in dart2js

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 0c03094e680e8590419a0c537798c054e9aef8c5..af365fe22dbf2111484dfd4f5c56275b14da04aa 100644
--- a/pkg/lookup_map/lib/lookup_map.dart
+++ b/pkg/lookup_map/lib/lookup_map.dart
@@ -7,7 +7,7 @@ library lookup_map;
 
 /// [LookupMap] is a simple, but very restricted map.  The map can only hold
 /// constant keys and the only way to use the map is to retrieve values with a
-/// key you already have.  Expect for lookup, any other operation in [Map] (like
+/// key you already have.  Except for lookup, any other operation in [Map] (like
 /// forEach, keys, values, length, etc) is not available.
 ///
 /// Constant [LookupMap]s are understood by dart2js and can be tree-shaken