Add an example core blocks category using the extension spec.

src/blocks/scratch3_core_example.js

@@ -0,0 +1,44 @@
+const BlockType = require('../extension-support/block-type');
+
+/**
+ * An example core block implemented using the extension spec.
+ * This is not loaded as part of the core blocks in the VM but it is provided
+ * and used as part of tests.
+ */
+class Scratch3CoreExample {
+    constructor (runtime) {
+        /**
+         * The runtime instantiating this block package.
+         * @type {Runtime}
+         */
+        this.runtime = runtime;
+    }
+
+    /**
+     * @returns {object} metadata for this extension and its blocks.
+     */
+    getInfo () {
+        return {
+            id: 'coreExample',
+            name: 'CoreEx', // This string does not need to be translated as this extension is only used as an example.
+            blocks: [
+                {
+                    opcode: 'exampleOpcode',
+                    blockType: BlockType.REPORTER,
+                    text: 'example block'
+                }
+            ]
+        };
+    }
+
+    /**
+     * Example opcode just returns the name of the stage target.
+     * @returns {string} The name of the first target in the project.
+     */
+    exampleOpcode () {
+        return this.runtime.getTargetForStage().getName();
+    }
+
+}
+
+module.exports = Scratch3CoreExample;