Copy of Flutter utility classes to make it simpler to later convert this code to Hummingbird. (#67)

* Copy of Flutter utility classes to make it simpler to write code with
dart:html and convert to hummingbird later.
There are no changes to these files other than stripping out portions of
files irrelevant on the web.

Author: jacob314

lib/ui/fake_flutter/assertions.dart

@@ -0,0 +1,39 @@
+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+part of 'fake_flutter.dart';
+
+/// Error class used to report Flutter-specific assertion failures and
+/// contract violations.
+class FlutterError extends AssertionError {
+  /// Creates a [FlutterError].
+  ///
+  /// See [message] for details on the format that the message should
+  /// take.
+  ///
+  /// Include as much detail as possible in the full error message,
+  /// including specifics about the state of the app that might be
+  /// relevant to debugging the error.
+  FlutterError(String message) : super(message);
+
+  /// The message associated with this error.
+  ///
+  /// The message may have newlines in it. The first line should be a terse
+  /// description of the error, e.g. "Incorrect GlobalKey usage" or "setState()
+  /// or markNeedsBuild() called during build". Subsequent lines should contain
+  /// substantial additional information, ideally sufficient to develop a
+  /// correct solution to the problem.
+  ///
+  /// In some cases, when a FlutterError is reported to the user, only the first
+  /// line is included. For example, Flutter will typically only fully report
+  /// the first exception at runtime, displaying only the first line of
+  /// subsequent errors.
+  ///
+  /// All sentences in the error should be correctly punctuated (i.e.,
+  /// do end the error message with a period).
+  @override
+  String get message => super.message;
+
+  @override
+  String toString() => message;
+}

lib/ui/fake_flutter/collections.dart

@@ -0,0 +1,49 @@
+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+library collections;
+
+// TODO(ianh): These should be on the Set and List classes themselves.
+
+/// Compares two sets for deep equality.
+///
+/// Returns true if the sets are both null, or if they are both non-null, have
+/// the same length, and contain the same members. Returns false otherwise.
+/// Order is not compared.
+///
+/// See also:
+///
+///   * [listEquals], which does something similar for lists.
+bool setEquals<T>(Set<T> a, Set<T> b) {
+  if (a == null)
+    return b == null;
+  if (b == null || a.length != b.length)
+    return false;
+  for (T value in a) {
+    if (!b.contains(value))
+      return false;
+  }
+  return true;
+}
+
+/// Compares two lists for deep equality.
+///
+/// Returns true if the lists are both null, or if they are both non-null, have
+/// the same length, and contain the same members in the same order. Returns
+/// false otherwise.
+///
+/// See also:
+///
+///   * [setEquals], which does something similar for sets.
+bool listEquals<T>(List<T> a, List<T> b) {
+  if (a == null)
+    return b == null;
+  if (b == null || a.length != b.length)
+    return false;
+  for (int index = 0; index < a.length; index += 1) {
+    if (a[index] != b[index])
+      return false;
+  }
+  return true;
+}