Use ES6 for code in extension docs

docs/extensions.md

@@ -9,8 +9,9 @@ wiki](https://github.com/LLK/docs/wiki/Extensions).
 ## Types of Extensions
 
 There are four types of extensions that can define everything from the Scratch's core library (such as the "Looks" and
-"Operators" categories) to unofficial extensions that can be loaded from a remote URL. Unofficial extensions are not
-yet supported.
+"Operators" categories) to unofficial extensions that can be loaded from a remote URL.
+
+**Scratch 3.0 does not yet support unofficial extensions.**
 
 For more details, see [this Extensions page on the wiki](https://github.com/LLK/docs/wiki/Extensions).
 
@@ -22,6 +23,15 @@ For more details, see [this Extensions page on the wiki](https://github.com/LLK/
 | Sandboxed                      | X    | X    | √        | √          |
 | Can save projects to community | √    | √    | √        | X          |
 
+## JavaScript Environment
+
+Most Scratch 3.0 is written using JavaScript features not yet commonly supported by browsers. For compatibility we
+transpile the code to ES5 before publishing or deploying. Any extension included in the `scratch-vm` repository may
+use ES6+ features and may use `require` to reference other code within the `scratch-vm` repository.
+
+Unofficial extensions must be self-contained. Authors of unofficial extensions are responsible for ensuring browser
+compatibility for those extensions, including transpiling if necessary.
+
 ## Translation
 
 Scratch extensions use the [ICU message format](http://userguide.icu-project.org/formatparse/messages) to handle
@@ -46,12 +56,17 @@ Scratch extensions are defined as a single Javascript class which accepts either
 across a well defined worker boundary (i.e. the sandbox).
 
 ```js
-var SomeBlocks = function (runtimeProxy) {
+class SomeBlocks {
+    constructor (runtime) {
         /**
-     * A proxy to communicate with the Scratch 3.0 runtime across a worker boundary.
+         * Store this for later communication with the Scratch VM runtime.
+         * If this extension is running in a sandbox then `runtime` is an async proxy object.
          * @type {Runtime}
          */
-    this.runtime = runtimeProxy;
+        this.runtime = runtime;
+    }
+
+    // ...
 }
 ```
 
@@ -59,7 +74,9 @@ All extensions must define a function called `getInfo` which returns an object t
 render both the blocks and the extension itself.
 
 ```js
-SomeBlocks.prototype.getInfo = function () {
+class SomeBlocks {
+    // ...
+    getInfo () {
         return {
             id: 'someBlocks',
             name: 'Some Blocks',
@@ -81,32 +98,40 @@ SomeBlocks.prototype.getInfo = function () {
                 }
             ]
         };
-};
+    }
+    // ...
+}
 ```
 
 Finally the extension must define a function for any "opcode" defined in the blocks. For example:
 
 ```js
-SomeBlocks.prototype.myReporter = function (args) {
+class SomeBlocks {
+    // ...
+    myReporter (args) {
         return args.TEXT.charAt(args.LETTER_NUM);
     };
+    // ...
+}
 ```
 
 ## Annotated Example
 
 ```js
-var SomeBlocks = function (runtimeProxy) {
+class SomeBlocks {
+    constructor (runtime) {
         /**
-     * A proxy to communicate with the Scratch 3.0 runtime across a worker boundary.
+         * Store this for later communication with the Scratch VM runtime.
+         * If this extension is running in a sandbox then `runtime` is an async proxy object.
          * @type {Runtime}
          */
-    this.runtime = runtimeProxy;
-};
+        this.runtime = runtime;
+    }
 
     /**
      * @return {object} This extension's metadata.
      */
-SomeBlocks.prototype.getInfo = function () {
+    getInfo () {
         return {
             // Required: the machine-readable name of this extension.
             // Will be used as the extension's namespace.
@@ -298,7 +323,7 @@ SomeBlocks.prototype.getInfo = function () {
      * @property {string} MY_ARG - the string value of the argument.
      * @returns {string} a string which includes the block argument value.
      */
-SomeBlocks.prototype.myReporter = function (args) {
+    myReporter (args) {
         // This message contains ICU placeholders, not Scratch placeholders
         const message = formatMessage({
             id: 'myReporter.result',
@@ -315,4 +340,5 @@ SomeBlocks.prototype.myReporter = function (args) {
             LETTER: result
         });
     };
+}
 ```