Add dummy dart:html library for documentation.

lib/html/doc/interface/Float32Array.dart

+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// WARNING: Do not edit - generated code.
+
+/// @domName Float32Array
+interface Float32Array extends ArrayBufferView, List<num> default _TypedArrayFactoryProvider {
+
+  Float32Array(int length);
+
+  Float32Array.fromList(List<num> list);
+
+  Float32Array.fromBuffer(ArrayBuffer buffer, [int byteOffset, int length]);
+
+  static final int BYTES_PER_ELEMENT = 4;
+
+  /** @domName Float32Array.length */
+  final int length;
+
+  /** @domName Float32Array.setElements */
+  void setElements(Object array, [int offset]);
+
+  /** @domName Float32Array.subarray */
+  Float32Array subarray(int start, [int end]);
+}