More String + elimination in tests

tests/corelib/reg_exp4_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.
 // Dart test for testing regular expressions in Dart.
 
 main() {
   try {
     RegExp ex = const RegExp(null);
     Expect.fail("Expected: NullPointerException got: no exception");
   } catch (Exception ex) {
     if (!(ex is NullPointerException)) {
-      Expect.fail("Expected: NullPointerException" + " got: ${ex}");
+      Expect.fail("Expected: NullPointerException got: ${ex}");
     }
   }
   try {
     const RegExp(@"^\w+$").hasMatch(null);
     Expect.fail("Expected: NullPointerException got: no exception");
   } catch (Exception ex) {
     if (!(ex is NullPointerException)) {
-      Expect.fail("Expected: NullPointerException" + " got: ${ex}");
+      Expect.fail("Expected: NullPointerException got: ${ex}");
     }
   }
   try {
     const RegExp(@"^\w+$").firstMatch(null);
     Expect.fail("Expected: NullPointerException got: no exception");
   } catch (Exception ex) {
     if (!(ex is NullPointerException)) {
-      Expect.fail("Expected: NullPointerException" + " got: ${ex}");
+      Expect.fail("Expected: NullPointerException got: ${ex}");
     }
   }
   try {
     const RegExp(@"^\w+$").allMatches(null);
     Expect.fail("Expected: NullPointerException got: no exception");
   } catch (Exception ex) {
     if (!(ex is NullPointerException)) {
-      Expect.fail("Expected: NullPointerException" + " got: ${ex}");
+      Expect.fail("Expected: NullPointerException got: ${ex}");
     }
   }
   try {
     const RegExp(@"^\w+$").stringMatch(null);
     Expect.fail("Expected: NullPointerException got: no exception");
   } catch (Exception ex) {
     if (!(ex is NullPointerException)) {
-      Expect.fail("Expected: NullPointerException" + " got: ${ex}");
+      Expect.fail("Expected: NullPointerException got: ${ex}");
     }
   }
 }