- Fix use of String operator + in core libraries.

runtime/lib/error.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.
 // Errors are created and thrown by DartVM only.
 // Changes here should also be reflected in corelib/error.dart as well
 
 class AssertionError {
   factory AssertionError._uninstantiable() {
     throw const UnsupportedOperationException(
         "AssertionError can only be allocated by the VM");
   }
   static _throwNew(int assertionStart, int assertionEnd)
       native "AssertionError_throwNew";
   String toString() {
-    return "'$url': Failed assertion: line $line pos $column: " +
+    return "'$url': Failed assertion: line $line pos $column: "
         "'$failedAssertion' is not true.";
   }
   final String failedAssertion;
   final String url;
   final int line;
   final int column;
 }
 
 class TypeError extends AssertionError {
   factory TypeError._uninstantiable() {
@@ skipping 41 matching lines @@
   final String _msg;
 }
 
 
 class StaticResolutionException implements Exception {
   factory StaticResolutionException._uninstantiable() {
     throw const UnsupportedOperationException(
         "StaticResolutionException can only be allocated by the VM");
   }
 
-  String toString() => "Unresolved static method: url '$url' line $line " +
+  String toString() => "Unresolved static method: url '$url' line $line "
       "pos $column\n$failedResolutionLine\n";
 
   static _throwNew(int token_pos) native "StaticResolutionException_throwNew";
 
   final String failedResolutionLine;
   final String url;
   final int line;
   final int column;
 }