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

utils/tests/string_encoding/unicode_core_tests.dart

Index: utils/tests/string_encoding/unicode_core_tests.dart
diff --git a/utils/tests/string_encoding/unicode_core_tests.dart b/utils/tests/string_encoding/unicode_core_tests.dart
index 1a02eb8d245872ee6a5e2698ece3c1d91e1f19af..0161d44bd1b53fe2650bb5f36c66c6b7bb153eab 100755
--- a/utils/tests/string_encoding/unicode_core_tests.dart
+++ b/utils/tests/string_encoding/unicode_core_tests.dart
@@ -27,6 +27,7 @@ class UnicodeCoreTests extends TestClass {
 
   void testCodepointsToUtf16CodeUnits() {
     // boundary conditions
+    Expect.listEquals([], codepointsToUtf16CodeUnits([]), "no input");
     Expect.listEquals([0x0], codepointsToUtf16CodeUnits([0x0]), "0");
     Expect.listEquals([0xd800, 0xdc00],
         codepointsToUtf16CodeUnits([0x10000]), "10000");
@@ -49,6 +50,7 @@ class UnicodeCoreTests extends TestClass {
 
   void testUtf16bytesToCodepoints() {
     // boundary conditions: First possible values
+    Expect.listEquals([], utf16CodeUnitsToCodepoints([]), "no input");
     Expect.listEquals([0x0], utf16CodeUnitsToCodepoints([0x0]), "0");
     Expect.listEquals([0x10000],
         utf16CodeUnitsToCodepoints([0xd800, 0xdc00]), "10000");