Fixes in smoke, which in turn should fix todomvc.

pkg/observe/lib/src/path_observer.dart

Index: pkg/observe/lib/src/path_observer.dart
diff --git a/pkg/observe/lib/src/path_observer.dart b/pkg/observe/lib/src/path_observer.dart
index 29af241490b928fd6b16469b710e0c9d6384ef77..be551024c0427482269a36282f8842ca73d8e90c 100644
--- a/pkg/observe/lib/src/path_observer.dart
+++ b/pkg/observe/lib/src/path_observer.dart
@@ -506,14 +506,13 @@ abstract class _Observer extends Bindable {
       throw new StateError('Observer has already been opened.');
     }
 
-    if (_minArgumentCount(callback) > _reportArgumentCount) {
+    if (smoke.minArgs(callback) > _reportArgumentCount) {
       throw new ArgumentError('callback should take $_reportArgumentCount or '
           'fewer arguments');
     }
 
     _notifyCallback = callback;
-    _notifyArgumentCount = min(_reportArgumentCount,
-        _maxArgumentCount(callback));
+    _notifyArgumentCount = min(_reportArgumentCount, smoke.maxArgs(callback));
 
     _connect();
     return _value;
@@ -560,27 +559,6 @@ abstract class _Observer extends Bindable {
   }
 }
 
-typedef _Func0();
-typedef _Func1(a);
-typedef _Func2(a, b);
-typedef _Func3(a, b, c);
-
-int _minArgumentCount(fn) {
-  if (fn is _Func0) return 0;
-  if (fn is _Func1) return 1;
-  if (fn is _Func2) return 2;
-  if (fn is _Func3) return 3;
-  return 4; // at least 4 arguments are required.
-}
-
-int _maxArgumentCount(fn) {
-  if (fn is _Func3) return 3;
-  if (fn is _Func2) return 2;
-  if (fn is _Func1) return 1;
-  if (fn is _Func0) return 0;
-  return -1;
-}
-
 class _ObservedSet {
   /// To prevent sequential [PathObserver]s and [CompoundObserver]s from
   /// observing the same object, we check if they are observing the same root