fix observe package to be strong mode clean

test/observe_test_utils.dart

Index: test/observe_test_utils.dart
diff --git a/test/observe_test_utils.dart b/test/observe_test_utils.dart
index 46d1b4646492e921e9947f60e41935df9f9e6384..31f2e9d515ed7b921606061ed377e770f73839ba 100644
--- a/test/observe_test_utils.dart
+++ b/test/observe_test_utils.dart
@@ -42,8 +42,10 @@ newMicrotask(_) => new Future.value();
 expectChanges(actual, expected, {reason}) =>
     expect('$actual', '$expected', reason: reason);
 
-List getListChangeRecords(List changes, int index) => changes
-    .where((c) => c.indexChanged(index)).toList();
+List<ListChangeRecord> getListChangeRecords(List changes, int index) =>
+    new List.from(changes.where((c) => c.indexChanged(index)));

[Siggi Cherem (dart-lang), 2016/03/25 17:36:29]

I'm surprised that you need to replace `.toList` with `new List.from` here.
Doesn't `toList()` preserve the type argument? 

[Jennifer Messerly, 2016/03/25 18:23:23]

they're different.

changes -- is probably a List<dynamic>
changes.toList() -- will give a List<dynamic> at runtime.

`new List.from` will infer as `new List<ListChangeRecord>.from()` which gives us
the explicit coercion we need to make this work.

[Siggi Cherem (dart-lang), 2016/03/25 18:30:01]

Ah - sorry, for a moment I thought we inferred List.from from the argument, but
we are inferring it from the declared return type of this function. I can see it
now.

 
-List getPropertyChangeRecords(List changes, Symbol property) => changes
-    .where((c) => c is PropertyChangeRecord && c.name == property).toList();
+List<PropertyChangeRecord> getPropertyChangeRecords(
+        List changes, Symbol property) =>
+    new List.from(
+        changes.where((c) => c is PropertyChangeRecord && c.name == property));