- Fix use of String operator + in language and corelib tests.

tests/corelib/src/RegExpAllMatchesTest.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 program for RegExp.allMatches.
 
 class RegExpAllMatchesTest {
   static testIterator() {
     var matches = new RegExp("foo").allMatches("foo foo");
     Iterator it = matches.iterator();
@@ skipping 13 matching lines @@
     Expect.equals(true, it.hasNext());
     Expect.equals(true, it2.hasNext());
     Expect.equals('foo', it.next().group(0));
     Expect.equals('foo', it2.next().group(0));
     Expect.equals(false, it.hasNext());
     Expect.equals(false, it2.hasNext());
   }
 
   static testForEach() {
     var matches = new RegExp("foo").allMatches("foo foo");
-    var str = "";
+    var strbuf = new StringBuffer();
     matches.forEach((Match m) {
-      str += m.group(0);
+      strbuf.add(m.group(0));
     });
-    Expect.equals("foofoo", str);
+    Expect.equals("foofoo", strbuf.toString());
   }
 
   static testMap() {
     var matches = new RegExp("foo?").allMatches("foo fo foo fo");
-    var mapped = matches.map((Match m) => m.group(0) + "bar");
+    var mapped = matches.map((Match m) => "${m.group(0)}bar");
     Expect.equals(4, mapped.length);
-    var str = "";
+    var strbuf = new StringBuffer();
     for (String s in mapped) {
-      str += s;
+      strbuf.add(s);
     }
-    Expect.equals("foobarfobarfoobarfobar", str);
+    Expect.equals("foobarfobarfoobarfobar", strbuf.toString());
   }
 
   static testFilter() {
     var matches = new RegExp("foo?").allMatches("foo fo foo fo");
     var filtered = matches.filter((Match m) {
       return m.group(0) == 'foo';
     });
     Expect.equals(2, filtered.length);
-    var str = "";
+    var strbuf = new StringBuffer();
     for (Match m in filtered) {
-      str += m.group(0);
+      strbuf.add(m.group(0));
     }
-    Expect.equals("foofoo", str);
+    Expect.equals("foofoo", strbuf.toString());
   }
 
   static testEvery() {
     var matches = new RegExp("foo?").allMatches("foo fo foo fo");
     Expect.equals(true, matches.every((Match m) {
       return m.group(0).startsWith("fo");
     }));
     Expect.equals(false, matches.every((Match m) {
       return m.group(0).startsWith("foo");
     }));
@@ skipping 34 matching lines @@
     testEvery();
     testSome();
     testIsEmpty();
     testGetCount();
   }
 }
 
 main() {
   RegExpAllMatchesTest.testMain();
 }