Replace bad_raw_string_negative_test with a multitest

tests/language/bad_raw_string_test.dart

+// Copyright (c) 2016, 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.
+
+main() {
+  // Raw String may not contain newline (may not be multi-line).
+  String x = ''
+    r'  /// 01: compile-time error
+'       /// 01: continued
+    r"  /// 02: compile-time error
+"       /// 02: continued
+    // Test that a raw string containing just one character, a \n char, fails.
+    // Enclose the test string in a bigger multiline string, except in case 03:
+    '''  /// 03: compile-time error
+    """
+    '''  /// 03: continued
+    r'
+'
+    '''  /// 03: continued
+    """
+    '''  /// 03: continued
+  ;
+}