diff --git a/.gitignore b/.gitignore
index 96bc6bb..e43b40f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,5 @@
 # Files and directories created by pub
+.dart_tool/
 .packages
 .pub/
 packages
diff --git a/.travis.yml b/.travis.yml
index 269df25..4f9b88c 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,9 +1,8 @@
 language: dart
 sudo: false
 dart:
-  - stable
   - dev
-  - 1.23.0
+  - 2.0.0-dev.23.0
 addons:
   # otherwise a number of tests in test/security/html_sanitizer_test.dart fail
   firefox: "latest"
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5288f3b..0c20ba4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,9 @@
-## 0.21.0+1-dev
+## 0.21.1-dev
 
-* Updated one of observable's tests to comply with dart 2 voidness semantics
+* Updated one test to comply with Dart 2 voidness semantics.
+* Fix Dart 2 runtime cast failure in `toObservable()`.
+* Loosen `ObservableList.from()` to take `Iterable`, not `Iterable<T>`. This
+  matches `List.from()` and avoids some unnecessary cast failures.
 
 ## 0.21.0
 
diff --git a/lib/src/observable_list.dart b/lib/src/observable_list.dart
index 2549f9b..b5ae6d3 100644
--- a/lib/src/observable_list.dart
+++ b/lib/src/observable_list.dart
@@ -42,8 +42,7 @@ class ObservableList<E> extends ListBase<E> with Observable {
 
   /// Creates an observable list with the elements of [other]. The order in
   /// the list will be the order provided by the iterator of [other].
-  factory ObservableList.from(Iterable<E> other) =>
-      new ObservableList<E>()..addAll(other);
+  ObservableList.from(Iterable other) : _list = new List<E>.from(other);
 
   /// The stream of summarized list changes, delivered asynchronously.
   ///
diff --git a/lib/src/to_observable.dart b/lib/src/to_observable.dart
index 3bf24b6..be83cb2 100644
--- a/lib/src/to_observable.dart
+++ b/lib/src/to_observable.dart
@@ -6,6 +6,8 @@ library observable.src.to_observable;
 
 import 'dart:collection';
 
+import 'package:dart_internal/extract_type_arguments.dart';
+
 import 'observable.dart' show Observable;
 import 'observable_list.dart' show ObservableList;
 import 'observable_map.dart' show ObservableMap;
@@ -30,22 +32,47 @@ toObservable(dynamic value, {bool deep: true}) =>
 
 dynamic _toObservableShallow(dynamic value) {
   if (value is Observable) return value;
-  if (value is Map) return new ObservableMap.from(value);
-  if (value is Iterable) return new ObservableList.from(value);
+
+  if (value is Map) {
+    return extractMapTypeArguments(value, <K, V>() {
+      var result = new ObservableMap<K, V>.createFromType(value);
+      value.forEach((k, v) {
+        result[_toObservableDeep(k)] = _toObservableDeep(v);
+      });
+      return result;
+    });
+  }
+
+  if (value is Iterable) {
+    return extractIterableTypeArgument(
+        value, <T>() => new ObservableList<T>.from(value));
+  }
+
   return value;
 }
 
 dynamic _toObservableDeep(dynamic value) {
   if (value is Observable) return value;
+
   if (value is Map) {
-    var result = new ObservableMap.createFromType(value);
-    value.forEach((k, v) {
-      result[_toObservableDeep(k)] = _toObservableDeep(v);
+    return extractMapTypeArguments(value, <K, V>() {
+      var result = new ObservableMap<K, V>.createFromType(value);
+      value.forEach((k, v) {
+        result[_toObservableDeep(k)] = _toObservableDeep(v);
+      });
+      return result;
     });
-    return result;
   }
+
   if (value is Iterable) {
-    return new ObservableList.from(value.map(_toObservableDeep));
+    return extractIterableTypeArgument(value, <T>() {
+      var result = new ObservableList<T>();
+      for (var element in value) {
+        result.add(_toObservableDeep(element));
+      }
+      return result;
+    });
   }
+
   return value;
 }
diff --git a/pubspec.yaml b/pubspec.yaml
index ea6c9e6..f64bcdc 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,12 +1,13 @@
 name: observable
-version: 0.21.0+1-dev
+version: 0.21.1-dev
 author: Dart Team <misc@dartlang.org>
 description: Support for marking objects as observable
 homepage: https://github.com/dart-lang/observable
 environment:
-  sdk: '>=1.23.0 <2.0.0'
+  sdk: '>=2.0.0-dev.23.0 <2.0.0'
 dependencies:
   collection: '^1.11.0'
+  dart_internal: '^0.1.1'
   meta: '^1.0.4'
   quiver: '>=0.24.0 <0.29.0'
 dev_dependencies: