Use native behavior for dead code elimination

sdk/lib/_internal/compiler/js_lib/js_number.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 super interceptor class for [JSInt] and [JSDouble]. The compiler
  * recognizes this class as an interceptor, and changes references to
  * [:this:] to actually use the receiver of the method, which is
@@ skipping 39 matching lines @@
   }
 
   bool get isFinite => JS('bool', r'isFinite(#)', this);
 
   num remainder(num b) {
     checkNull(b); // TODO(ngeoffray): This is not specified but co19 tests it.
     if (b is! num) throw new ArgumentError(b);
     return JS('num', r'# % #', this, b);
   }
 
-  num abs() => JS('num', r'Math.abs(#)', this);
+  num abs() => JS('returns:num;effects:none;depends:none;throws:never',
+      r'Math.abs(#)', this);
 
   num get sign => this > 0 ? 1 : this < 0 ? -1 : this;
 
   static const int _MIN_INT32 = -0x80000000;
   static const int _MAX_INT32 = 0x7FFFFFFF;
 
   int toInt() {
     if (this >= _MIN_INT32 && this <= _MAX_INT32) {
       return JS('int', '# | 0', this);
     }
@@ skipping 386 matching lines @@
 }
 
 class JSDouble extends JSNumber implements double {
   const JSDouble();
   Type get runtimeType => double;
 }
 
 class JSPositiveInt extends JSInt {}
 class JSUInt32 extends JSPositiveInt {}
 class JSUInt31 extends JSUInt32 {}