Removed unnecessary string concatenations

runtime/observatory/lib/src/elements/metric/graph.dart

 // Copyright (c) 2016, 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';
 import 'dart:async';
 import 'package:observatory/models.dart' as M;
 import 'package:charted/charted.dart';
 import 'package:observatory/src/elements/helpers/rendering_scheduler.dart';
 import 'package:observatory/src/elements/helpers/tag.dart';
@@ skipping 53 matching lines @@
     final min = _metrics.getMinValue(_isolate, _metric);
     final max = _metrics.getMaxValue(_isolate, _metric);
     final rows = _metrics
         .getSamples(_isolate, _metric)
         .map((s) => [s.time.millisecondsSinceEpoch, s.value])
         .toList();
     final current = rows.last.last;
 
     var message = 'current: $current';
     if (min != null) {
-      message = 'min: $min, ' + message;
+      message = 'min: $min, $message';
     }
     if (max != null) {
       message = message + ', max: $max';
     }
 
     final host = new DivElement();
     children = [
       new DivElement()
         ..classes = ['memberList']
         ..children = [
@@ skipping 40 matching lines @@
       return;
     }
     final rect = host.getBoundingClientRect();
     var series = new ChartSeries("one", [1], new LineChartRenderer());
     var config = new ChartConfig([series], [0]);
     config.minimumSize = new Rect(rect.width, rect.height);
     final data = new ChartData(_columns, rows);
     new CartesianArea(host, data, config, state: new ChartState()).draw();
   }
 }