Remove uses of DartObjectImpl

lib/src/codegen/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.
 
 library dev_compiler.src.codegen.side_effect_analysis;
 
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/constant.dart';
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/error.dart' show ErrorReporter;
@@ skipping 91 matching lines @@
             new ErrorReporter(
                 new RecordingErrorListener(), unit.element.source));
 
   // TODO(jmesserly): this is used to determine if the field initialization is
   // side effect free. We should make the check more general, as things like
   // list/map literals/regexp are also side effect free and fairly common
   // to use as field initializers.
   bool isFieldInitConstant(VariableDeclaration field) =>
       field.initializer == null || computeConstant(field) != null;
 
-  DartObjectImpl computeConstant(VariableDeclaration field) {
+  DartObject computeConstant(VariableDeclaration field) {
     // If the constant is already computed by ConstantEvaluator, just return it.
     VariableElementImpl element = field.element;
     var result = element.evaluationResult;
     if (result != null) return result.value;
 
     // 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);
   }
 }