Add toStructuredText to ConstantExpression and align method names with ConstantValue

tests/compiler/dart2js/mirrors_used_test.dart

Index: tests/compiler/dart2js/mirrors_used_test.dart
diff --git a/tests/compiler/dart2js/mirrors_used_test.dart b/tests/compiler/dart2js/mirrors_used_test.dart
index 08442738fd52ab2dc39c3d747724179077e42059..a96cf2add0c4a91ed04e56f8e09d8da1c0230716 100644
--- a/tests/compiler/dart2js/mirrors_used_test.dart
+++ b/tests/compiler/dart2js/mirrors_used_test.dart
@@ -137,7 +137,7 @@ void main() {
     for (var dependency in backend.metadataConstants) {
       ConstantValue constant = dependency.constant;
       Expect.isFalse(compiledConstants.contains(constant),
-                     constant.toStructuredString());
+                     constant.toStructuredText());
     }
 
     // The type literal 'Foo' is both used as metadata, and as a plain value in