Re-apply "Move the handling of operator[] into the new interceptors."

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of _interceptors;
 
 /**
  * The interceptor class for [String]. The compiler recognizes this
  * class as an interceptor, and changes references to [:this:] to
  * actually use the receiver of the method, which is generated as an extra
@@ skipping 145 matching lines @@
       hash = JS('int', '# ^ (# >> 6)', hash, hash);
     }
     hash = 0x1fffffff & (hash + ((0x03ffffff & hash) <<  3));
     hash = JS('int', '# ^ (# >> 11)', hash, hash);
     return 0x1fffffff & (hash + ((0x00003fff & hash) << 15));
   }
 
   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);
+  }
 }