restructure string decoding to support iterable use and include benchmarks for UTF-8 decoding.

utils/tests/string_encoding/dunit.dart

 #!/usr/bin/env dart
 // Copyright (c) 2012, 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("dunit");
 
 typedef void Test();
 typedef TestResult SynchTest();
 typedef Future<TestResult> AsynchTest();
@@ skipping 68 matching lines @@
 
 class FailedTest extends _ExceptionResult implements TestResult {
   FailedTest(String testDescription, var exception) :
       super(testDescription, exception);
 
   String toString() => ">>> Test failure in ${_testDescription} " +
       "with:\n${exception}\n";
 }
 
 class TestError extends _ExceptionResult implements TestResult {
-  TestError(String testDescription, var exception) :
+  TestError(String testDescription, var exception, var this.stacktrace) :
       super(testDescription, exception);
 
   String toString() => ">>> Test error caught in " +
-      "${_testDescription} with:\n${exception}\n";
+      "${_testDescription} with:\n${exception}\n$stacktrace\n";
+
+  var stacktrace;
 }
 
 class TestClass {
   void register(String description, Function test, TestSuite suite) {
     suite._registerTest(TestResult _() {
       setUp();
       try {
         test();
         tearDown();
         return new PassedTest(description);
       } catch (ExpectException x) {
         tearDown();
         return new FailedTest(description, x);
-      } catch (var x) {
+      } catch (var x, var stacktrace) {
         tearDown();
-        return new TestError(description, x);
+        return new TestError(description, x, stacktrace);
       }
     });
   }
 
   abstract void registerTests(TestSuite suite);
   void setUp() {}
   void tearDown() {}
 }