Add #library tag to please the testing setup we use on the build bots.

tests/corelib/math_test.dart

 // Copyright (c) 2011, 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.
 
 // We temporarily test both the new math library and the old Math
 // class. This can easily be simplified once we get rid of the Math
 // class entirely.
+#library('math_test');
 #import('dart:math', prefix: 'math');
 
 class MathTest {
   static void testConstants() {
     // Source for mathematical constants is Wolfram Alpha.
     Expect.equals(2.7182818284590452353602874713526624977572470936999595749669,
                   Math.E);
     Expect.equals(2.3025850929940456840179914546843642076011014886287729760333,
                   Math.LN10);
     Expect.equals(0.6931471805599453094172321214581765680755001343602552541206,
@@ skipping 486 matching lines @@
     testExp();
     testPow();
     testParseInt();
   }
 }
 
 main() {
   MathTest.testMain();
   MathLibraryTest.testMain();
 }