optimize null safe ops by using DDC's let abstraction

pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/operations.dart

 // Copyright (c) 2015, 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.
 
 /// This library defines runtime operations on objects used by the code
 /// generator.
 part of dart._runtime;
 
 class InvocationImpl extends Invocation {
   final Symbol memberName;
@@ skipping 732 matching lines @@
 // tools.
 stackPrint(exception) {
   var error = recordJsError(exception);
   JS('', 'console.log(#.stack ? #.stack : "No stack trace for: " + #)', error,
       error, error);
 }
 
 // Forward to dart:_js_helper to create a _StackTrace object.
 stackTrace(exception) => getTraceFromException(exception);
 
-///
-/// Implements a sequence of .? operations.
-///
-/// Will call each successive callback, unless one returns null, which stops
-/// the sequence.
-///
-nullSafe(obj, @rest callbacks) => JS(
-    '',
-    '''(() => {
-  if ($obj == null) return $obj;
-  for (let callback of $callbacks) {
-    $obj = callback($obj);
-    if ($obj == null) break;
-  }
-  return $obj;
-})()''');
-
 final _value = JS('', 'Symbol("_value")');
 
 ///
 /// Looks up a sequence of [keys] in [map], recursively, and
 /// returns the result. If the value is not found, [valueFn] will be called to
 /// add it. For example:
 ///
 ///     let map = new Map();
 ///     putIfAbsent(map, [1, 2, 'hi ', 'there '], () => 'world');
 ///
@@ skipping 232 matching lines @@
     JS('', '# = "+" + #', name, name);
   }
   return name;
 }
 
 /// Emulates the implicit "loadLibrary" function provided by a deferred library.
 ///
 /// Libraries are not actually deferred in DDC, so this just returns a future
 /// that completes immediately.
 Future loadLibrary() => new Future.value();