Remove use of deprecated mirror API from pkg/observe.

pkg/observe/lib/src/observable.dart

Index: pkg/observe/lib/src/observable.dart
diff --git a/pkg/observe/lib/src/observable.dart b/pkg/observe/lib/src/observable.dart
index f798f4446f46c541c25579890e93a1f8ea859a54..35e91c8912e989ececca48a1e89f0f74c384c686 100644
--- a/pkg/observe/lib/src/observable.dart
+++ b/pkg/observe/lib/src/observable.dart
@@ -82,8 +82,11 @@ abstract class Observable {
     // actually includes this mixin. While perhaps too inclusive, it lets us
     // avoid complex logic that walks "with" and "implements" clauses.
     for (var type = mirror.type; type != objectType; type = type.superclass) {
-      for (var field in type.variables.values) {
-        if (field.isFinal || field.isStatic || field.isPrivate) continue;
+      for (var field in type.declarations.values) {
+        if (field is! VariableMirror ||
+            field.isFinal ||
+            field.isStatic ||
+            field.isPrivate) continue;
 
         for (var meta in field.metadata) {
           if (meta.reflectee is ObservableProperty) {