Fix strong mode warnings.

lib/src/canonicalized_map.dart

Index: lib/src/canonicalized_map.dart
diff --git a/lib/src/canonicalized_map.dart b/lib/src/canonicalized_map.dart
index d967f70d005b5fafd981b4755a4e18b93438b1bf..cc78105eb18a926ade4d1742720715551154a140 100644
--- a/lib/src/canonicalized_map.dart
+++ b/lib/src/canonicalized_map.dart
@@ -6,6 +6,10 @@ import 'dart:collection';
 
 import 'utils.dart';
 
+typedef C _Canonicalize<C, K>(K key);
+
+typedef bool _IsValidKey(Object key);
+
 /// A map whose keys are converted to canonical values of type `C`.
 ///
 /// This is useful for using case-insensitive String keys, for example. It's
@@ -16,9 +20,9 @@ import 'utils.dart';
 /// By default, `null` is allowed as a key. It can be forbidden via the
 /// `isValidKey` parameter.
 class CanonicalizedMap<C, K, V> implements Map<K, V> {
-  final Function _canonicalize;
+  final _Canonicalize<C, K> _canonicalize;
 
-  final Function _isValidKeyFn;
+  final _IsValidKey _isValidKeyFn;
 
   final _base = new Map<C, Pair<K, V>>();
 
@@ -52,7 +56,7 @@ class CanonicalizedMap<C, K, V> implements Map<K, V> {
 
   V operator [](Object key) {
     if (!_isValidKey(key)) return null;
-    var pair = _base[_canonicalize(key)];
+    var pair = _base[_canonicalize(key as K)];
     return pair == null ? null : pair.last;
   }
 
@@ -71,7 +75,7 @@ class CanonicalizedMap<C, K, V> implements Map<K, V> {
 
   bool containsKey(Object key) {
     if (!_isValidKey(key)) return false;
-    return _base.containsKey(_canonicalize(key));
+    return _base.containsKey(_canonicalize(key as K));
   }
 
   bool containsValue(Object value) =>
@@ -96,7 +100,7 @@ class CanonicalizedMap<C, K, V> implements Map<K, V> {
 
   V remove(Object key) {
     if (!_isValidKey(key)) return null;
-    var pair = _base.remove(_canonicalize(key));
+    var pair = _base.remove(_canonicalize(key as K));
     return pair == null ? null : pair.last;
   }