Moving ListBase, FixedLengthListMixin and UmodifiableListMixin to collection.

runtime/lib/string_patch.dart

Index: runtime/lib/string_patch.dart
diff --git a/runtime/lib/string_patch.dart b/runtime/lib/string_patch.dart
index 1ba5e1a919b43b74d7d4800ca746f5ba47c2d67b..43101b0e47580695298a33a27c167640876e4aeb 100644
--- a/runtime/lib/string_patch.dart
+++ b/runtime/lib/string_patch.dart
@@ -417,7 +417,7 @@ class _StringBase {
     return result;
   }
 
-  List<int> get codeUnits => new CodeUnits(this);
+  List<int> get codeUnits => new _CodeUnits(this);
 
   Runes get runes => new Runes(this);
 
@@ -613,3 +613,17 @@ class _StringMatch implements Match {
   final String str;
   final String pattern;
 }
+
+/**
+ * An [Iterable] of the UTF-16 code units of a [String] in index order.
+ */
+class _CodeUnits extends Object with ListMixin<int>,
+                                     UnmodifiableListMixin<int> {
+  /** The string that this is the code units of. */
+  String _string;
+
+  _CodeUnits(this._string);
+
+  int get length => _string.length;
+  int operator[](int i) => _string.codeUnitAt(i);
+}