Use native behavior for dead code elimination

sdk/lib/_internal/compiler/js_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 141 matching lines @@
     checkInt(startIndex);
     if (endIndex == null) endIndex = length;
     checkInt(endIndex);
     if (startIndex < 0 ) throw new RangeError.value(startIndex);
     if (startIndex > endIndex) throw new RangeError.value(startIndex);
     if (endIndex > length) throw new RangeError.value(endIndex);
     return JS('String', r'#.substring(#, #)', this, startIndex, endIndex);
   }
 
   String toLowerCase() {
-    return JS('String', r'#.toLowerCase()', this);
+    return JS(
+        'returns:String;effects:none;depends:none;throws:null(1)',
+        r'#.toLowerCase()', this);
   }
 
   String toUpperCase() {
-    return JS('String', r'#.toUpperCase()', this);
+    return JS(
+        'returns:String;effects:none;depends:none;throws:null(1)',
+        r'#.toUpperCase()', this);
   }
 
   // Characters with Whitespace property (Unicode 6.2).
   // 0009..000D    ; White_Space # Cc       <control-0009>..<control-000D>
   // 0020          ; White_Space # Zs       SPACE
   // 0085          ; White_Space # Cc       <control-0085>
   // 00A0          ; White_Space # Zs       NO-BREAK SPACE
   // 1680          ; White_Space # Zs       OGHAM SPACE MARK
   // 180E          ; White_Space # Zs       MONGOLIAN VOWEL SEPARATOR
   // 2000..200A    ; White_Space # Zs       EN QUAD..HAIR SPACE
@@ skipping 250 matching lines @@
     return stringContainsUnchecked(this, other, startIndex);
   }
 
   bool get isEmpty => length == 0;
 
   bool get isNotEmpty => !isEmpty;
 
   int compareTo(String other) {
     if (other is !String) throw new ArgumentError(other);
     return this == other ? 0
-      : JS('bool', r'# < #', this, other) ? -1 : 1;
+        : JS('bool', r'# < #', this, other) ? -1 : 1;
   }
 
   // Note: if you change this, also change the function [S].
   String toString() => this;
 
   /**
    * This is the [Jenkins hash function][1] but using masking to keep
    * values in SMI range.
    *
    * [1]: http://en.wikipedia.org/wiki/Jenkins_hash_function
@@ skipping 15 matching lines @@
   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);
   }
 }