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

tools/testing/legpad/legpad.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.
 
 #import("dart:html", prefix:"html");
 #import('../../../lib/compiler/compiler.dart', prefix: "compiler_lib");
 #import('../../../lib/uri/uri.dart', prefix:"uri_lib");
 
 /**
  * This is the entrypoint for a version of the leg compiler that
  * runs in the browser.
  *
  * Because this is running in the browser, we do not access
  * the file system.  Instead, we assume that all necessary files
  * have been placed in inert <script> elements somewhere on the html page.
  *
  * (See legpad.py for more details on how the html page is constructed.)
  */
 void main() {
   new Legpad().run();
 }
 
 class Legpad {
   // id of script element containing name of the main dart file
   // to compile
   static final String MAIN_ID = "main_id";
 
-  Legpad() : warnings = "" {}
+  Legpad() : warnings = new StringBuffer();
 
   // accumulates diagnostic messages emitted by the leg compiler
-  String warnings;
+  StringBuffer warnings;
 
   // the generated javascript
   String output;
 
   String readAll(String filename) {
     String text = getText(idOfFilename(filename));
     print("read $filename (${text.length} bytes)");
     return text;
   }
 
   void diagnosticHandler(uri_lib.Uri uri, int begin, int end,
                                  String message, bool fatal) {
-    StringBuffer sb = new StringBuffer();
-    sb.add(message);    
+    warnings.add(message);
     if (uri !== null) {
-      sb.add(" (${uri.toString()}: $begin, $end)");
+      warnings.add(" ($uri: $begin, $end)");
     }
     if (fatal) {
-      sb.add(" (fatal)");
+      warnings.add(" (fatal)");
     }
-    sb.add("\n");
-    warnings += sb.toString();
+    warnings.add("\n");
   }
 
   Future<String> readUriFromString(uri_lib.Uri uri) {
     Completer<String> completer = new Completer<String>();
     completer.complete(readAll(uri.toString()));
     return completer.future;
   }
 
   /**
    * Returns the id of the <script> element that contains
    * the contents of this file.  (Replace all slashes
    * and dots in the file name with underscores.)
    */
   String idOfFilename(String filename) {
     return filename.replaceAll("/", "_").replaceAll(".", "_");
   }
 
   void run() {
     String mainFile = getText(MAIN_ID);
     setText("input", readAll(mainFile));
     Stopwatch stopwatch = new Stopwatch.start();
     runLeg();
     int elapsedMillis = stopwatch.elapsedInMs();
     if (output === null) {
       output = "throw 'dart2js compilation error';\n";
     }
 
     setText("output", output);
-    setText("warnings", warnings);
+    setText("warnings", warnings.toString());
     int lineCount = lineCount(output);
-    String timing = "generated $lineCount lines " + 
+    String timing = "generated $lineCount lines " +
         "(${output.length} characters) in " +
         "${((elapsedMillis) / 1000).toStringAsPrecision(3)} seconds";
     setText("timing", timing);
   }
-  
+
   static int lineCount(String s) {
     Iterable<Match> matches = "\n".allMatches(s);
     int n = 0;
     for (Match m in matches) {
       n++;
     }
     return n;
-  } 
+  }
 
   void runLeg() {
     uri_lib.Uri mainUri = new uri_lib.Uri.fromString(getText(MAIN_ID));
     uri_lib.Uri libraryRoot = new uri_lib.Uri.fromString("dartdir/");
     List<String> compilerArgs = [
       "--enable_type_checks",
       "--enable_asserts"
     ];
 
     // TODO(mattsh): dart2js api should be synchronous
@@ skipping 21 matching lines @@
     }
     return element.text.trim();
   }
 
   // TODO(mattsh): should exist in standard lib somewhere
   static String htmlEscape(String text) {
     return text.replaceAll('&', '&amp;').replaceAll(
         '>', '&gt;').replaceAll('<', '&lt;');
   }
 }