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

tests/language/closure_type_test.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.
 // Dart test for a closure result type test that cannot be eliminated at compile
 // time.
 
+#library('closure_type_test');
+#import('dart:math', prefix: 'math');
+
+class Math {
+  static int sqrt(x) => math.sqrt(x);
+}
+
 isCheckedMode() {
   try {
     var i = 1;
     String s = i;
     return false;
   } catch(var e) {
     return true;
   }
 }
 
@@ skipping 11 matching lines @@
     Expect.equals(value, x * x);
   } catch (TypeError error) {
     got_type_error = true;
   }
   // Type error expected in checked mode only.
   Expect.isTrue(got_type_error == isCheckedMode());
 }
 
 root(x) => Math.sqrt(x);
 
-main() => test(root, 4);
+main() => test(root, 4);