Move the handling of operator[] into the new interceptors.

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

Index: sdk/lib/_internal/compiler/implementation/lib/js_string.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/lib/js_string.dart	(revision 15907)
+++ sdk/lib/_internal/compiler/implementation/lib/js_string.dart	(working copy)
@@ -163,4 +163,10 @@
   Type get runtimeType => String;
 
   int get length => JS('int', r'#.length', this);
+
+  String operator [](int index) {
+    if (index is !int) throw new ArgumentError(index);
+    if (index >= length || index < 0) throw new RangeError.value(index);
+    return JS('String', '#[#]', this, index);
+  }
 }