content/static: example playground output fixes

The example playgrounds now clear the existing output before
printing new output when the Run button is pressed. Ensures
the loading message is cleared when example code produces no
output. Additionally, the output box will now display for all
examples because they are editable. Examples that don't
initially have output can be changed to produce output.

Change-Id: Ie70f98ab505a28705e2ea65869bddb84ef6927ec
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/295192
Trust: Jamal Carvalho <jamal@golang.org>
Run-TryBot: Jamal Carvalho <jamal@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Julie Qiu <julie@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>

content/static/html/doc/example.tmpl

@@ -14,9 +14,7 @@
       <p><a class="Documentation-examplesPlay" href="{{.}}">Open in Go playground »</a></p>{{"\n" -}}
       {{- end -}}
       {{render_code .Example}}{{"\n" -}}
-      {{- if (or .Output .EmptyOutput) -}}
-        <pre><span class="Documentation-exampleOutputLabel">Output:</span>{{"\n\n"}}<span class="Documentation-exampleOutput">{{- .Output -}}</span></pre>{{"\n" -}}
-      {{- end -}}
+      <pre><span class="Documentation-exampleOutputLabel">Output:</span>{{"\n\n"}}<span class="Documentation-exampleOutput">{{- .Output -}}</span></pre>{{"\n" -}}
     </div>{{"\n" -}}
     {{- if .Play -}}
       <div class="Documentation-exampleButtonsContainer">

content/static/js/playground.js

@@ -118,6 +118,7 @@ export class PlaygroundExampleController {
         if (Errors) {
           this.setOutputText(Errors);
         }
+        this.setOutputText('');
         for (const e of Events || []) {
           this.setOutputText(e.Message);
           await new Promise(resolve => setTimeout(resolve, e.Delay / 1000000));

content/static/js/playground.ts

@@ -231,6 +231,7 @@ export class PlaygroundExampleController {
         if (Errors) {
           this.setOutputText(Errors);
         }
+        this.setOutputText('');
         for (const e of Events || []) {
           this.setOutputText(e.Message);
           await new Promise(resolve => setTimeout(resolve, e.Delay / 1000000));