Refactor 'dart:profiler' UserTag API

tests/lib/profiler/user_tags_test.dart

Index: tests/lib/profiler/user_tags_test.dart
diff --git a/tests/lib/profiler/user_tags_test.dart b/tests/lib/profiler/user_tags_test.dart
index 4070d90daff26701436133ee300e99420eb8fccf..f09abcdd2acc641c81f81d09a51b524eb9538b21 100644
--- a/tests/lib/profiler/user_tags_test.dart
+++ b/tests/lib/profiler/user_tags_test.dart
@@ -6,6 +6,11 @@
 import 'dart:profiler';
 import 'package:expect/expect.dart';
 
+// Test that the default tag is set.
+testDefault() {
+  Expect.isTrue(identical(UserTag.defaultTag, getCurrentTag()));
+}
+
 // Test that the label property matches the constructor.
 void testLabel() {
   var label = 'Hello World';
@@ -19,6 +24,9 @@ void testCanonicalize(tag1) {
   var label = 'Global Tag';
   var tag = new UserTag(label);
   Expect.isTrue(identical(tag, tag1));
+  var defaultLabel = 'Default';
+  var defaultTag = new UserTag(defaultLabel);
+  Expect.isTrue(identical(UserTag.defaultTag, defaultTag));
 }
 
 
@@ -29,16 +37,6 @@ void testMakeCurrent(tag) {
 }
 
 
-// Test clearCurrentTag.
-void testClearCurrent(tag) {
-  tag.makeCurrent();
-  Expect.isTrue(identical(tag, getCurrentTag()));
-  var old_tag = clearCurrentTag();
-  Expect.isTrue(identical(tag, old_tag));
-  Expect.isNull(getCurrentTag());
-}
-
-
 // Test that we reach a limit of tags an exception is thrown.
 void testExhaust() {
   var i = 0;
@@ -50,7 +48,29 @@ void testExhaust() {
 }
 
 
+var callerTag = new UserTag('caller');
+var calleeTag = new UserTag('callee');
+
+void callee() {
+  var old = calleeTag.makeCurrent();
+  Expect.isTrue(identical(calleeTag, getCurrentTag()));
+  old.makeCurrent();
+}
+
+void testCallerPattern() {
+  Expect.isTrue(identical(UserTag.defaultTag, getCurrentTag()));
+  var old = callerTag.makeCurrent();
+  Expect.isTrue(identical(callerTag, getCurrentTag()));
+  callee();
+  Expect.isTrue(identical(callerTag, getCurrentTag()));
+  old.makeCurrent();
+  Expect.isTrue(identical(UserTag.defaultTag, getCurrentTag()));
+}
+
+
 main() {
+  testDefault();
+  testCallerPattern();
   var label = 'Global Tag';
   var tag = new UserTag(label);
   testLabel();
@@ -58,6 +78,5 @@ main() {
   for (var i = 0; i < 2000; i++) {
     testMakeCurrent(tag);
   }
-  testClearCurrent(tag);
   Expect.throws(testExhaust);
 }