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

utils/tests/string_encoding/unicode_tests.dart

Index: utils/tests/string_encoding/unicode_tests.dart
diff --git a/utils/tests/string_encoding/unicode_tests.dart b/utils/tests/string_encoding/unicode_tests.dart
index 53114e4b27c5502eaecfaa1b07c71cf9d481b3b7..7188fe1ca820f7078e7a5ec91956e0bf1b03a31b 100755
--- a/utils/tests/string_encoding/unicode_tests.dart
+++ b/utils/tests/string_encoding/unicode_tests.dart
@@ -25,6 +25,8 @@ class UnicodeTests extends TestClass {
   void registerTests(TestSuite suite) {
     register("testCodepointsToString", testCodepointsToString, suite);
     register("testStringToCodepoints", testStringToCodepoints, suite);
+    register("testEmptyCodepointsToString", testEmptyCodepointsToString, suite);
+    register("testEmptyStringToCodepoints", testEmptyStringToCodepoints, suite);
   }
 
   void testStringToCodepoints() {
@@ -34,4 +36,12 @@ class UnicodeTests extends TestClass {
   void testCodepointsToString() {
     Expect.stringEquals(testPhrase, codepointsToString(testCodepoints));
   }
+
+  void testEmptyCodepointsToString() {
+    Expect.stringEquals("", codepointsToString(<int>[]));
+  }
+
+  void testEmptyStringToCodepoints() {
+    Expect.listEquals([], stringToCodepoints(""));
+  }
 }