Create IterableMixinWorkaround and move most of the Collections methods there.

samples/swarm/DataSource.dart

Index: samples/swarm/DataSource.dart
diff --git a/samples/swarm/DataSource.dart b/samples/swarm/DataSource.dart
index e471567e19311fc8613c2191b40123bbb781dc75..6bda0f598c13d8965fb209706c49450854fb7f17 100644
--- a/samples/swarm/DataSource.dart
+++ b/samples/swarm/DataSource.dart
@@ -90,9 +90,9 @@ class Sections extends Collection<Section> {
   List<Section> get sections => _sections;
 
   // Collection<Section> methods:
-  bool every(bool f(Section element)) => Collections.every(this, f);
-  bool any(bool f(Section element)) => Collections.any(this, f);
-  void forEach(void f(Section element)) { Collections.forEach(this, f); }
+  bool every(bool f(Section element)) => IterableMixinWorkaround.every(this, f);
+  bool any(bool f(Section element)) => IterableMixinWorkaround.any(this, f);
+  void forEach(void f(Section element)) { IterableMixinWorkaround.forEach(this, f); }
 
   // TODO(jmesserly): this should be a property
   bool get isEmpty => length == 0;