Bug 1568143 - Don't use JsTerm reference to evaluate code. r=jdescottes.

JSTerm `execute` function is renamed to `_execute` to emphasize
that it's a private function.
A `dispatchEvaluateExpression` is added to the WebConsoleWrapper
so other panels can use that if they need to.
A test helper is added to be able to evaluate without grabbing
the jsterm reference, and tests are modified to use the new
helper, or the existing `executeAndWaitForMessage`.

Differential Revision: https://phabricator.services.mozilla.com/D39024

devtools/client/inspector/markup/markup-context-menu.js

@@ -344,11 +344,8 @@ class MarkupContextMenu {
    */
   _showDOMProperties() {
     this.toolbox.openSplitConsole().then(() => {
-      const panel = this.toolbox.getPanel("webconsole");
-      const jsterm = panel.hud.jsterm;
-
-      jsterm.execute("inspect($0)");
-      jsterm.focus();
+      const { hud } = this.toolbox.getPanel("webconsole");
+      hud.ui.wrapper.dispatchEvaluateExpression("inspect($0)");
     });
   }