Updating snippets documentation README.md (#50483)

Updates the README file for snippets.

dev/snippets/README.md

 # Dartdoc Generation
 
-The Flutter API documentation contains code blocks that help provide
-context or a good starting point when learning to use any of Flutter's APIs.
+The Flutter API documentation contains code blocks that help provide context or
+a good starting point when learning to use any of Flutter's APIs.
 
 To generate these code blocks, Flutter uses dartdoc tools to turn documentation
 in the source code into API documentation, as seen on https://api.flutter.dev/.
@@ -16,13 +16,23 @@ in the source code into API documentation, as seen on https://api.flutter.dev/.
 
 ## Types of code blocks
 
-There's two kinds of code blocks.
+There are three kinds of code blocks.
 
-* snippets, which are more or less context-free code snippets that we
+* A `snippet`, which is a more or less context-free code snippet that we
   magically determine how to analyze, and
 
-* samples, which get placed into a full-fledged application, and can
-  be actually executed inline in the documentation using DartPad.
+* A `dartpad` sample, which gets placed into a full-fledged application, and can
+  be actually executed inline in the documentation on the web page using
+  DartPad.
+
+* A `sample`, which gets placed into a full-fledged application, but isn't
+  placed into DartPad in the documentation because it doesn't make sense to do
+  so.
+
+Ideally every sample is a DartPad sample, but some samples don't have any visual
+representation, and some just don't make sense that way (for example, sample
+code for setting the system UI's notification area color on Android won't do
+anything on the web).
 
 ### Snippet Tool
 
@@ -45,41 +55,39 @@ code. Here is an example of the code `snippet` tool in use:
 /// {@end-tool}
 ```
 
-This will generate sample code that can be copied to the clipboard and added
-to existing applications.
+This will generate sample code that can be copied to the clipboard and added to
+existing applications.
 
-This uses the skeleton for [snippet](config/skeletons/snippet.html)
-snippets when generating the HTML to put into the Dart docs.
+This uses the skeleton for [snippet](config/skeletons/snippet.html) snippets
+when generating the HTML to put into the Dart docs.
 
 #### Analysis
 
-The `../bots/analyze-sample-code.dart` script finds code inside the
-`@tool snippet` sections and uses the Dart analyzer to check them.
+The `../bots/analyze-sample-code.dart` script finds code inside the `@tool
+snippet` sections and uses the Dart analyzer to check them.
 
 There are several kinds of sample code you can specify:
 
-* Constructor calls, typically showing what might exist in a build
-  method. These will be inserted into an assignment expression
-  assigning to a variable of type "dynamic" and followed by a
-  semicolon, for the purposes of analysis.
+* Constructor calls, typically showing what might exist in a build method. These
+  will be inserted into an assignment expression assigning to a variable of type
+  "dynamic" and followed by a semicolon, for the purposes of analysis.
 
-* Class definitions. These start with "class", and are analyzed
-  verbatim.
+* Class definitions. These start with "class", and are analyzed verbatim.
 
-* Other code. It gets included verbatim, though any line that says
-  `// ...` is considered to separate the block into multiple blocks
-  to be processed individually.
+* Other code. It gets included verbatim, though any line that says `// ...` is
+  considered to separate the block into multiple blocks to be processed
+  individually.
 
-The above means that it's tricky to include verbatim imperative code
-(e.g. a call to a method), since it won't be valid to have such code
-at the top level. Instead, wrap it in a function or even a whole
-class, or make it a valid variable declaration.
+The above means that it's tricky to include verbatim imperative code (e.g. a
+call to a method), since it won't be valid to have such code at the top level.
+Instead, wrap it in a function or even a whole class, or make it a valid
+variable declaration.
 
-You can declare code that should be included in the analysis but not
-shown in the API docs by adding a comment "// Examples can assume:" to
-the file (usually at the top of the file, after the imports),
-following by one or more commented-out lines of code. That code is
-included verbatim in the analysis. For example:
+You can declare code that should be included in the analysis but not shown in
+the API docs by adding a comment "// Examples can assume:" to the file (usually
+at the top of the file, after the imports), following by one or more
+commented-out lines of code. That code is included verbatim in the analysis. For
+example:
 
 ```dart
 // Examples can assume:
@@ -95,9 +103,12 @@ You can assume that the entire Flutter framework and most common
 
 ![Code sample image](assets/code_sample.png)
 
-The code `sample` tool can expand sample code into full Flutter applications.
-These sample applications can be directly copied and used to demonstrate the
-API's functionality in a sample application:
+The code `sample` and `dartpad` tools can expand sample code into full Flutter
+applications. These sample applications can be directly copied and used to
+demonstrate the API's functionality in a sample application, or used with the
+`flutter create` command to create a local project with the sample code. The
+`dartpad` samples are embedded into the API docs web page and are live
+applications in the API documentation.
 
 ```dart
 /// {@tool sample --template=stateless_widget_material}
@@ -129,31 +140,34 @@ API's functionality in a sample application:
 This uses the skeleton for [application](config/skeletons/sample.html)
 snippets.
 
-Code `sample` also allow for quick Flutter app generation using the following command:
+The `sample` and `dartpad` tools also allow for quick Flutter app generation
+using the following command:
 
 ```bash
 flutter create --sample=[directory.File.sampleNumber] [name_of_project_directory]
 ```
 
+This command is displayed as part of the sample in the API docs.
+
 #### Templates
 
-In order to support showing an entire app when you click on the right tab of
-the code sample UI, we have to be able to insert the `sample` block into the
-template and instantiate the right parts.
+In order to support showing an entire app when you click on the right tab of the
+code sample UI, we have to be able to insert the `sample` or `dartpad` block
+into the template and instantiate the right parts.
 
 To do this, there is a [config/templates](config/templates) directory that
 contains a list of templates. These templates represent an entire app that the
-`sample` can be placed into, basically a replacement for `lib/main.dart` in a
-flutter app package.
+`sample` or `dartpad` can be placed into, basically a replacement for
+`lib/main.dart` in a flutter app package.
 
 For more information about how to create, use, or update templates, see
 [config/templates/README.md](config/templates/README.md).
 
 #### Analysis
 
-The `../bots/analyze-sample-code.dart` script finds code inside the
-`@tool sample` sections and uses the Dart analyzer to check them
-after applying the specified template.
+The `../bots/analyze-sample-code.dart` script finds code inside the `@tool
+sample` sections and uses the Dart analyzer to check them after applying the
+specified template.
 
 ## Skeletons
 
@@ -161,7 +175,8 @@ A skeleton (in relation to this tool) is an HTML template into which the Dart
 code blocks and descriptions are interpolated.
 
 There is currently one skeleton for
-[application](config/skeletons/sample.html) samples and one for
+[application](config/skeletons/sample.html) samples, one for
+[dartpad](config/skeletons/dartpad-sample.html) and one for
 [snippet](config/skeletons/snippet.html) code samples, but there could be more.
 
 Skeletons use mustache notation (e.g. `{{code}}`) to mark where components will
@@ -184,8 +199,30 @@ that your code blocks are showing up correctly:
 
 1. Make an update to a code block or create a new code block.
 2. From the root directory, run `./dev/bots/docs.sh`. This should start
-generating a local copy of the API documentation.
+   generating a local copy of the API documentation.
 3. Once complete, check `./dev/docs/doc` to check your API documentation. The
-search bar will not work locally, so open `./dev/docs/doc/index.html` to
-navigate through the documentation, or search `./dev/docs/doc/flutter` for your
-page of interest.
+   search bar will not work locally, so open `./dev/docs/doc/index.html` to
+   navigate through the documentation, or search `./dev/docs/doc/flutter` for
+   your page of interest.
+
+Note that generating the sample output will not allow you to run your code in
+DartPad, because DartPad pulls the code it runs from the appropriate docs server
+(master or stable).
+
+Copy the generated code and paste it into a regular DartPad instance to test if
+it runs in DartPad. To get the code that will be produced by your documentation
+changes, run sample analysis locally (see the next section) and paste the output
+into a DartPad at https://dartpad.dartlang.org.
+
+## Running sample analysis locally
+
+If all you want to do is analyze the sample code you have written locally, then
+generating the entire docs output takes a long time.
+
+Instead, you can run the analysis locally with this command from the Flutter root:
+
+```
+TMPDIR=/tmp bin/cache/dart-sdk/bin/dart dev/bots/analyze-sample-code.dart --temp=samples
+```
+
+This will analyze the samples, and leave the output in /tmp/samples