Fix gr-diff-builder tests in Polymer 2

Polymer 2 doesn't support binding in test-fixture (see
PolymerElements/test-fixture#47).
Another problem that pluginLayers can be set null - additional
check was added.

Change-Id: I48c3fbaab6763867bd872ba10507a6dbd7ddf732

polygerrit-ui/app/elements/diff/gr-diff-builder/gr-diff-builder.html

@@ -227,8 +227,10 @@
             this.$.coverageLayerLeft,
             this.$.coverageLayerRight,
           ];
-          layers.push(...this.pluginLayers);
 
+          if (this.pluginLayers) {
+            layers.push(...this.pluginLayers);
+          }
           this._layers = layers;
         },
 

polygerrit-ui/app/elements/diff/gr-diff-builder/gr-diff-builder_test.html

@@ -37,7 +37,7 @@
 
 <test-fixture id="basic">
   <template is="dom-template">
-    <gr-diff-builder plugin-layers="[[pluginLayers]]">
+    <gr-diff-builder>
       <table id="diffTable"></table>
     </gr-diff-builder>
   </template>
@@ -596,7 +596,8 @@
       let withPluginLayerCount;
       setup(() => {
         const pluginLayers = [];
-        element = fixture('basic', {pluginLayers});
+        element = fixture('basic');
+        element.pluginLayers = pluginLayers;
         element._showTrailingWhitespace = true;
         element._setupAnnotationLayers();
         initialLayersCount = element._layers.length;
@@ -610,7 +611,8 @@
       suite('with plugin layers', () => {
         const pluginLayers = [{}, {}];
         setup(() => {
-          element = fixture('basic', {pluginLayers});
+          element = fixture('basic');
+          element.pluginLayers = pluginLayers;
           element._showTrailingWhitespace = true;
           element._setupAnnotationLayers();
           withPluginLayerCount = element._layers.length;