Add toStructuredText to ConstantExpression and align method names with ConstantValue

tests/compiler/dart2js/constant_expression_evaluate_test.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 dart2js.constants.expressions.evaluate_test;
 
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 import 'package:compiler/src/constants/evaluation.dart';
@@ skipping 209 matching lines @@
   Compiler compiler = result.compiler;
   var library = compiler.mainApp;
   constants.forEach((String name, ConstantData data) {
     FieldElement field = library.localLookup(name);
     ConstantExpression constant = field.constant;
     data.expectedValues.forEach(
         (Map<String, String> env, String expectedText) {
       Environment environment = new MemoryEnvironment(compiler, env);
       ConstantValue value =
           constant.evaluate(environment, DART_CONSTANT_SYSTEM);
-      String valueText = value.toStructuredString();
+      String valueText = value.toStructuredText();
       Expect.equals(expectedText, valueText,
           "Unexpected value '${valueText}' for contant "
-          "`${constant.getText()}`, expected '${expectedText}'.");
+          "`${constant.toDartText()}`, expected '${expectedText}'.");
     });
   });
 }