Add Dart class for _ExternalUint8ClampedArray and API to allocate it.

runtime/lib/byte_array.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.
 
 patch class Int8List {
   /* patch */ factory Int8List(int length) {
     return new _Int8Array(length);
   }
 
   /* patch */ factory Int8List.transferable(int length) {
@@ skipping 1313 matching lines @@
     return _length() * _BYTES_PER_ELEMENT;
   }
 
   static const int _BYTES_PER_ELEMENT = 1;
 
   int _getIndexed(int index) native "ExternalUint8Array_getIndexed";
   int _setIndexed(int index, int value) native "ExternalUint8Array_setIndexed";
 }
 
 
+class _ExternalUint8ClampedArray
+  extends _ByteArrayBase implements Uint8ClampedList {
+
+  int operator[](int index) {
+    return _getIndexed(index);
+  }
+
+  int operator[]=(int index, int value) {
+    _setIndexed(index, _toClampedUint8(value));
+  }
+
+  Iterator<int> get iterator {
+    return new _ByteArrayIterator<int>(this);
+  }
+
+  List<int> getRange(int start, int length) {
+    _rangeCheck(this.length, start, length);
+    List<int> result = new Uint8ClampedList(length);
+    result.setRange(0, length, this, start);
+    return result;
+  }
+
+  void setRange(int start, int length, List<int> from, [int startFrom = 0]) {
+    if (from is _ExternalUint8ClampedArray || from is _Uint8ClampedArray) {
+      _setRange(start * _BYTES_PER_ELEMENT,
+                length * _BYTES_PER_ELEMENT,
+                from,
+                startFrom * _BYTES_PER_ELEMENT);
+    } else {
+      Arrays.copy(from, startFrom, this, start, length);
+    }
+  }
+
+  String toString() {
+    return Collections.collectionToString(this);
+  }
+
+  int bytesPerElement() {
+    return _BYTES_PER_ELEMENT;
+  }
+
+  int lengthInBytes() {
+    return _length() * _BYTES_PER_ELEMENT;
+  }
+
+  static const int _BYTES_PER_ELEMENT = 1;
+
+  int _getIndexed(int index) native "ExternalUint8ClampedArray_getIndexed";
+  int _setIndexed(int index, int value) native "ExternalUint8ClampedArray_setIndexed";
+}
+
+
 class _ExternalInt16Array extends _ByteArrayBase implements Int16List {
   int operator[](int index) {
     return _getIndexed(index);
   }
 
   int operator[]=(int index, int value) {
     _setIndexed(index, _toInt16(value));
   }
 
   Iterator<int> get iterator {
@@ skipping 1253 matching lines @@
   ByteArray asByteArray([int start = 0, int length]) {
     if (length == null) {
       length = this.lengthInBytes();
     }
     _rangeCheck(this.length, start, length);
     return _array.subByteArray(_offset + start, length);
   }
 
   static const int _BYTES_PER_ELEMENT = 8;
 }