Moving ListBase, FixedLengthListMixin and UmodifiableListMixin to collection.

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 395f970950757c552d77e0d0ae35d88f1c005f08..e6184c0d893fc9fe19a4db0f3bc1b97c00464754 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/js_string.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/js_string.dart
@@ -137,7 +137,7 @@ class JSString extends Interceptor implements String, JSIndexable {
     return JS('String', r'#.trim()', this);
   }
 
-  List<int> get codeUnits => new CodeUnits(this);
+  List<int> get codeUnits => new _CodeUnits(this);
 
   Runes get runes => new Runes(this);
 
@@ -211,3 +211,16 @@ class JSString extends Interceptor implements String, JSIndexable {
     return JS('String', '#[#]', this, index);
   }
 }
+
+/**
+ * An [Iterable] of the UTF-16 code units of a [String] in index order.
+ */
+class _CodeUnits extends UnmodifiableListBase<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);
+}