Rename String.concat to operator+.

sdk/lib/_internal/compiler/implementation/tree/dartstring.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.
 
 part of tree;
 
 /**
  * The [DartString] type represents a Dart string value as a sequence of Unicode
  * Scalar Values.
  * After parsing, any valid [LiteralString] will contain a [DartString]
@@ skipping 106 matching lines @@
   String toStringCache;
   ConsDartString(DartString left, DartString right)
       : this.left = left,
         this.right = right,
         length = left.length + right.length;
 
   Iterator<int> get iterator => new ConsDartStringIterator(this);
 
   String slowToString() {
     if (toStringCache != null) return toStringCache;
-    toStringCache = left.slowToString().concat(right.slowToString());
+    toStringCache = left.slowToString() + right.slowToString();
     return toStringCache;
   }
   SourceString get source => new StringWrapper(slowToString());
 }
 
 class ConsDartStringIterator implements Iterator<int> {
   HasNextIterator<int> currentIterator;
   DartString right;
   bool hasNextLookAhead;
   int _current = null;
@@ skipping 88 matching lines @@
         }
         _current = value;
         break;
       default:
         _current = code;
     }
     return true;
   }
 }