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

utils/tests/string_encoding/unicode_tests.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("unicode_tests");
 #import("dunit.dart");
 #import("../../string_encoding/unicode.dart");
 
 void main() {
   TestSuite suite = new TestSuite();
   suite.registerTestClass(new UnicodeTests());
   suite.run();
 }
 
 class UnicodeTests extends TestClass {
   static final String testPhrase =
       "The quick brown fox jumps over the lazy dog.";
 
   static final List<int> testCodepoints = const<int> [
       84, 104, 101, 32, 113, 117, 105, 99, 107, 32, 98, 114, 111, 119, 110,
       32, 102, 111, 120, 32, 106, 117, 109, 112, 115, 32, 111, 118, 101, 114,
       32, 116, 104, 101, 32, 108, 97, 122, 121, 32, 100, 111, 103, 46];
 
   void registerTests(TestSuite suite) {
     register("testCodepointsToString", testCodepointsToString, suite);
     register("testStringToCodepoints", testStringToCodepoints, suite);
+    register("testEmptyCodepointsToString", testEmptyCodepointsToString, suite);
+    register("testEmptyStringToCodepoints", testEmptyStringToCodepoints, suite);
   }
 
   void testStringToCodepoints() {
     Expect.listEquals(testCodepoints, stringToCodepoints(testPhrase));
   }
 
   void testCodepointsToString() {
     Expect.stringEquals(testPhrase, codepointsToString(testCodepoints));
   }
+
+  void testEmptyCodepointsToString() {
+    Expect.stringEquals("", codepointsToString(<int>[]));
+  }
+
+  void testEmptyStringToCodepoints() {
+    Expect.listEquals([], stringToCodepoints(""));
+  }
 }