Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/token_naming_test.dart

 // Copyright (c) 2015, 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.
 
 import "package:compiler/src/js_backend/js_backend.dart" show TokenScope;
 
 import "package:expect/expect.dart";
 
 String forwardN(TokenScope scope, int N) {
   for (int i = 1; i < N; ++i) {
     scope.getNextName();
   }
 
   return scope.getNextName();
 }
 
-int main() {
+void main() {
   // Test a normal scope.
   TokenScope scope = new TokenScope();
 
   // We start with 'a'.
   Expect.equals("a", scope.getNextName());
   // We have 24 lower case characters, as s and g are illegal.
   Expect.equals("A", forwardN(scope, 24));
   // Make it overflow by skipping all uppercase.
   Expect.equals("a_", forwardN(scope, 26));
   // Now numbers.
@@ skipping 26 matching lines @@
   Expect.equals("A", forwardN(scope, 22));
   // Make it overflow by skipping all uppercase.
   Expect.equals("a_", forwardN(scope, 26));
   // Now numbers.
   Expect.equals("a0", forwardN(scope, 1));
   Expect.equals("a9", forwardN(scope, 9));
   // Make sure 'aa' is skipped on wrapping
   Expect.equals("ab", forwardN(scope, 1));
   Expect.equals("az", forwardN(scope, 24));
 }