optimize null safe ops by using DDC's let abstraction

pkg/dev_compiler/lib/src/compiler/side_effect_analysis.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.
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/generated/constant.dart';
 import 'package:analyzer/error/listener.dart'
     show AnalysisErrorListener, ErrorReporter;
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/source.dart' show Source;
 import 'package:analyzer/src/dart/ast/ast.dart';
 
 /// True is the expression can be evaluated multiple times without causing
 /// code execution. This is true for final fields. This can be true for local
 /// variables, if:
-/// * they are not assigned within the [context].
-/// * they are not assigned in a function closure anywhere.
-/// True is the expression can be evaluated multiple times without causing
-/// code execution. This is true for final fields. This can be true for local
-/// variables, if:
 ///
 /// * they are not assigned within the [context] scope.
 /// * they are not assigned in a function closure anywhere.
 ///
 /// This method is used to avoid creating temporaries in cases where we know
 /// we can safely re-evaluate [node] multiple times in [context]. This lets
 /// us generate prettier code.
 ///
 /// This method is conservative: it should never return `true` unless it is
 /// certain the [node] is stateless, because generated code may rely on the
@@ skipping 97 matching lines @@
     // ConstantEvaluator will not compute constants for non-const fields,
     // so run ConstantVisitor for those to figure out if the initializer is
     // actually a constant (and therefore side effect free to evaluate).
     assert(!field.isConst);
 
     var initializer = field.initializer;
     if (initializer == null) return null;
     return initializer.accept(constantVisitor);
   }
 }