Implement creation of ByteData objects using the Dart API.

runtime/lib/typeddata.dart

Index: runtime/lib/typeddata.dart
===================================================================
--- runtime/lib/typeddata.dart	(revision 20386)
+++ runtime/lib/typeddata.dart	(working copy)
@@ -228,12 +228,12 @@
 patch class ByteData {
   /* patch */ factory ByteData(int length) {
     var list = new _Uint8Array(length);
-    return new _ByteDataView(list.buffer);
+    return new _ByteDataView(list.buffer, 0, length);
   }
 
   /* patch */ factory ByteData.transferable(int length) {
     var list = new _Uint8Array.transferable(length);
-    return new _ByteDataView(list.buffer);
+    return new _ByteDataView(list.buffer, 0, length);
   }
 
   /* patch */ factory ByteData.view(ByteBuffer buffer,