Adapt String interface for Utf16.

sdk/lib/_internal/compiler/implementation/lib/js_string.dart

Index: sdk/lib/_internal/compiler/implementation/lib/js_string.dart
diff --git a/sdk/lib/_internal/compiler/implementation/lib/js_string.dart b/sdk/lib/_internal/compiler/implementation/lib/js_string.dart
index d0d857c2feb5996c99446a27bf1c90fcb659af97..381a690564ac2c86ee075b4d996308cbc0bd2828 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/js_string.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/js_string.dart
@@ -13,7 +13,9 @@ part of _interceptors;
 class JSString implements String {
   const JSString();
 
-  int charCodeAt(index) {
+  int charCodeAt(index) => codeUnitAt(index);
+
+  int codeUnitAt(int index) {
     if (index is !num) throw new ArgumentError(index);
     if (index < 0) throw new RangeError.value(index);
     if (index >= length) throw new RangeError.value(index);
@@ -147,6 +149,14 @@ class JSString implements String {
     return result;
   }
 
+  Iterable<int> get codeUnits {
+    throw new UnimplementedError("String.codeUnits");
+  }
+
+  Iterable<int> get runes {
+    throw new UnimplementedError("String.runes");
+  }
+
   int indexOf(String other, [int start = 0]) {
     checkNull(other);
     if (start is !int) throw new ArgumentError(start);