Remove methods on StackTrace class.

tests/language/stack_trace_test.dart

 // Copyright (c) 2011, 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.
 // Dart test program for testing throw statement
 
 import "package:expect/expect.dart";
 
 class MyException {
   const MyException(String message) : message_ = message;
   final String message_;
 }
 
 class Helper {
   static int f1(int i) {
     try {
       i = func();
       i = 10;
     } on MyException catch (exception, stacktrace) {
       i = 50;
       print(exception.message_);
-      Expect.equals((stacktrace != null), true);
+      Expect.isNotNull(stacktrace);
       print(stacktrace);
     }
     try {
       int j;
       i = func1();
       i = 200;
     } on MyException catch (exception, stacktrace) {
       i = 50;
       print(exception.message_);
-      Expect.equals((stacktrace != null), true);
+      Expect.isNotNull(stacktrace);
       print(stacktrace);
     }
     try {
       int j;
       i = func2();
       i = 200;
     } on MyException catch (exception, stacktrace) {
       i = 50;
       print(exception.message_);
-      Expect.equals((stacktrace != null), true);
+      Expect.isNotNull(stacktrace);
       print(stacktrace);
     } finally {
       i = i + 800;
     }
     return i;
   }
 
   static int func() {
     int i = 0;
     while (i < 10) {
@@ skipping 27 matching lines @@
 
 class StackTraceTest {
   static testMain() {
     Expect.equals(850, Helper.f1(1));
   }
 }
 
 main() {
   StackTraceTest.testMain();
 }