Remove string concatenation with + from all Dart files in tools directory.

tools/testing/dart/multitest.dart

 // Copyright (c) 2012, 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.
 
 #library("multitest");
 
 #import("dart:io");
 #import("test_suite.dart");
 
 // Multitests are Dart test scripts containing lines of the form
@@ skipping 73 matching lines @@
   // time we see a multitest line with a new key.
   Map<String, List<String>> testsAsLines = new Map<String, List<String>>();
 
   int lineCount = 0;
   for (String line in lines) {
     lineCount++;
     var annotation = new _Annotation.from(line);
     if (annotation != null) {
       testsAsLines.putIfAbsent(annotation.key,
           () => new List<String>.from(testTemplate)).add(line);
-      outcomes.putIfAbsent(annotation.key, 
+      outcomes.putIfAbsent(annotation.key,
           () => new Set<String>());
       if (annotation.rest == 'continued') {
         continue;
       } else {
         for (String nextOutcome in annotation.outcomesList) {
-          outcomes[annotation.key].add(nextOutcome);  
+          outcomes[annotation.key].add(nextOutcome);
           if (!validMultitestOutcomes.contains(nextOutcome)) {
             // TODO(zundel): fix long line
             Expect.fail(
               "Invalid test directive '$nextOutcome' on line ${lineCount}: ${annotation.rest} ");
           }
         }
       }
     } else {
       testTemplate.add(line);
       for (var test in testsAsLines.getValues()) test.add(line);
     }
   }
 
   // Check that every key (other than the none case) has at least one outcome
   for (var outcomeKey in outcomes.getKeys()) {
     if (outcomeKey != 'none' && outcomes[outcomeKey].isEmpty()) {
       // TODO(zundel): fix long line
       Expect.fail("Test ${outcomeKey} has no valid annotated outcomes. Expected one of: ${validMultitestOutcomes.toString()}");
     }
   }
 
   // Add the template, with no multitest lines, as a test with key 'none'.
   testsAsLines['none'] = testTemplate;
   outcomes['none'] = new Set<String>();
 
   // Copy all the tests into the output map tests, as multiline strings.
   for (String key in testsAsLines.getKeys()) {
     tests[key] =
-        Strings.join(testsAsLines[key], line_separator) + line_separator;
+        Strings.join(testsAsLines[key], line_separator).concat(line_separator);
   }
 }
 
 // Represents a mutlitest annotation in the special /// comment.
 class _Annotation {
   String key;
   String rest;
   List<String> outcomesList;
   _Annotation() {}
   factory _Annotation.from(String line) {
@@ skipping 128 matching lines @@
     // TestSuite.forDirectory.
     split.removeLast();
   }
   String path = '${generatedTestDir.path}/${split.last()}';
   Directory dir = new Directory(path);
   if (!dir.existsSync()) {
     dir.createSync();
   }
   return path;
 }