Deprecate Math object in corelib in favor of dart:math library.

samples/third_party/dromaeo/tests/dom-modify-dom.dart

 #library("dom_modify");
 #import("dart:dom_deprecated");
 #import("dart:json");
+#import("dart:math", prefix: "Math");
 #source("Common.dart");
 #source("RunnerSuite.dart");
 
 void main() {
   final int num = 400;
 
   String str = 'null';
   // Very ugly way to build up the string, but let's mimic JS version as much as possible.
   for (int i = 0; i < 1024; i++) {
     str = "$str${new String.fromCharCodes([((25 * Math.random()) + 97).toInt()])}";
@@ skipping 45 matching lines @@
     .test('appendChild', () {
       for (int i = 0; i < elems.length; i++)
         document.body.appendChild(elems[i]);
     })
     .test('insertBefore', () {
       for (int i = 0; i < elems.length; i++)
         document.body.insertBefore(elems[i], document.body.firstChild);
     })
     .end();
 }