Fix the return type of String.split in DDC to be correct

tests/corelib_2/string_split_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.
 
 import "package:expect/expect.dart";
 
 main() {
   testSplitString();
   testSplitRegExp();
   testSplitPattern();
 }
 
-testSplit(List expect, String string, Pattern pattern) {
+testSplit(List<String> expect, String string, Pattern pattern) {
   String patternString;
   if (pattern is String) {
     patternString = '"$pattern"';
   } else if (pattern is RegExp) {
     patternString = "/${pattern.pattern}/";
   } else {
     patternString = pattern.toString();
   }
-  Expect.listEquals(
-      expect, string.split(pattern), '"$string".split($patternString)');
+  List actual = string.split(pattern);
+
+  // Check that the list is growable/mutable
+  actual
+    ..add('42')
+    ..removeLast();
+
+  // Ensure that the correct type is reified.
+  actual = actual as List<String>;
+  Expect.throws(() {
+    actual.add(42);
+  }, (e) => e is TypeError, 'List<String>.add should not accept an int');
+
+  Expect.listEquals(expect, actual, '"$string".split($patternString)');
 }
 
 /** String patterns. */
 void testSplitString() {
   // Normal match.
   testSplit(["a", "b", "c"], "a b c", " ");
   testSplit(["a", "b", "c"], "adbdc", "d");
   testSplit(["a", "b", "c"], "addbddc", "dd");
   // No match.
   testSplit(["abc"], "abc", " ");
@@ skipping 92 matching lines @@
   final regexp;
   RegExpWrap(String source) : regexp = new RegExp(source);
   Iterable<Match> allMatches(String string, [int start = 0]) =>
       regexp.allMatches(string, start);
 
   Match matchAsPrefix(String string, [int start = 0]) =>
       regexp.matchAsPrefix(string, start);
 
   String toString() => "Wrap(/${regexp.pattern}/)";
 }