Remove string + uses from dart:html.

lib/html/src/shared_FactoryProviders.dart

Index: lib/html/src/shared_FactoryProviders.dart
diff --git a/lib/html/src/shared_FactoryProviders.dart b/lib/html/src/shared_FactoryProviders.dart
index e9ea9238de2f682b10f3e8331f4ff1b90c25dbed..81a4bb40e89244a100d6ffb623158b06ee2fd756 100644
--- a/lib/html/src/shared_FactoryProviders.dart
+++ b/lib/html/src/shared_FactoryProviders.dart
@@ -90,7 +90,8 @@ class _SVGElementFactoryProvider {
     parentTag.innerHTML = svg;
     if (parentTag.elements.length == 1) return parentTag.nodes.removeLast();
 
-    throw new IllegalArgumentException('SVG had ${parentTag.elements.length} ' +
+    throw new IllegalArgumentException(
+        'SVG had ${parentTag.elements.length} '
         'top-level elements but 1 expected');
   }
 }