graph: fix spurious error modal on page load (tensorflow#2651)

Summary:
Fixes tensorflow#2646, and fixes tensorflow#2650 as a side effect.

I’ve gone with an additional boolean field `_datasetsFetched` rather
than making `_datasets` nullable because `_datasets` is also passed down
directly to `tf-graph-controls`, which expects a non-null array. This
seemed simpler, all things considered.

Test Plan:

 1. Launch TensorBoard. Navigate to <http://localhost:6006/#graphs>.
 2. Observe that there is no flash of “No graph definition files were
    found” (though there is instead a brief flash of white).
 3. Note that the list of graphs loads, the first graph is selected, and
    the fragment updates.
 4. Refresh the page. Note that the page behaves as in the initial load:
    in particular, there is no error modal.
 5. Change the fragment such that the run is invalid (`#graphs&run=bad`)
    and note that the error modal appears without refreshing the page.
 6. Refresh the page, and note that the error modal appears, and the
    first graph is automatically selected and loaded underneath it.

wchargin-branch: graph-fix-error-modal

tensorboard/plugins/graph/tf_graph_dashboard/tf-graph-dashboard.html

@@ -38,7 +38,10 @@
 <dom-module id="tf-graph-dashboard">
   <template>
     <paper-dialog id="error-dialog" with-backdrop></paper-dialog>
-    <template is="dom-if" if="[[_datasetsEmpty(_datasets)]]">
+    <template
+      is="dom-if"
+      if="[[_datasetsState(_datasetsFetched, _datasets, 'EMPTY')]]"
+    >
       <div style="max-width: 540px; margin: 80px auto 0 auto;">
         <h3>No graph definition files were found.</h3>
         <p>
@@ -75,7 +78,10 @@ <h3>No graph definition files were found.</h3>
         </p>
       </div>
     </template>
-    <template is="dom-if" if="[[!_datasetsEmpty(_datasets)]]">
+    <template
+      is="dom-if"
+      if="[[!_datasetsState(datasetsFetched, _datasets, 'PRESENT')]]"
+    >
       <tf-dashboard-layout>
         <tf-graph-controls
           id="controls"
@@ -191,6 +197,10 @@ <h3>No graph definition files were found.</h3>
         type: Array,
         value: () => [],
       },
+      _datasetsFetched: {
+        type: Boolean,
+        value: false,
+      },
       _selectedDataset: {
         type: Number,
         value: 0,
@@ -275,8 +285,8 @@ <h3>No graph definition files were found.</h3>
       '_maybeFetchHealthPills(_debuggerDataEnabled, allStepsModeEnabled, ' +
         'specificHealthPillStep, _selectedNode)',
       '_maybeInitializeDashboard(_isAttached)',
-      '_determineSelectedDataset(_datasets, run)',
-      '_updateSelectedDatasetName(_datasets, _selectedDataset)',
+      '_determineSelectedDataset(_datasetsFetched, _datasets, run)',
+      '_updateSelectedDatasetName(_datasetsFetched, _datasets, _selectedDataset)',
     ],
     attached: function() {
       this.set('_isAttached', true);
@@ -366,7 +376,7 @@ <h3>No graph definition files were found.</h3>
         this._debuggerDataEnabled &&
         this.healthPillsToggledOn &&
         this._renderHierarchy &&
-        !this._datasetsEmpty(this._datasets)
+        this._datasetsState(this._datasetsFetched, this._datasets, 'PRESENT')
       );
     },
     _maybeInitializeDashboard: function(isAttached) {
@@ -421,9 +431,10 @@ <h3>No graph definition files were found.</h3>
 
             return {name: runName, tags: tagsWithV1Graph};
           });
+        this._datasetsFetched = true;
       });
     },
-    _determineSelectedDataset(datasets, run) {
+    _determineSelectedDataset(datasetsFetched, datasets, run) {
       // By default, load the first dataset.
       if (!run) {
         // By default, load the first dataset.
@@ -434,17 +445,20 @@ <h3>No graph definition files were found.</h3>
       // If the URL specifies a dataset, load it.
       const dataset = datasets.findIndex((d) => d.name === run);
       if (dataset === -1) {
-        // Tell the user if the dataset cannot be found to avoid misleading
-        // the user.
-        const dialog = this.$$('#error-dialog');
-        dialog.textContent = `No dataset named "${run}" could be found.`;
-        dialog.open();
+        if (datasetsFetched) {
+          // Tell the user if the dataset cannot be found to avoid misleading
+          // the user.
+          const dialog = this.$$('#error-dialog');
+          dialog.textContent = `No dataset named "${run}" could be found.`;
+          dialog.open();
+        }
         return;
       }
 
       this.set('_selectedDataset', dataset);
     },
-    _updateSelectedDatasetName(datasets, selectedDataset) {
+    _updateSelectedDatasetName(datasetsFetched, datasets, selectedDataset) {
+      if (!datasetsFetched) return;
       // Cannot update `run` to update the hash in case datasets for graph is empty.
       if (datasets.length <= selectedDataset) return;
       this.set('run', datasets[selectedDataset].name);
@@ -528,8 +542,10 @@ <h3>No graph definition files were found.</h3>
         }.bind(this)
       );
     },
-    _datasetsEmpty: function(datasets) {
-      return !datasets || !datasets.length;
+    _datasetsState: function(datasetsFetched, datasets, state) {
+      if (!datasetsFetched) return state === 'NOT_LOADED';
+      if (!datasets || !datasets.length) return state === 'EMPTY';
+      return state === 'PRESENT';
     },
     _renderHierarchyChanged: function(renderHierarchy) {
       // Reload any data on the graph when the render hierarchy (which determines which nodes are