Update nnbd_migration README with more details.

* Use title case on all headings.
* Link to https://dart.dev/null-safety and
  https://dart.dev/tools/pub/cmd/pub-outdated
* Remove any text which suggests to publish. Official docs warn
  strongly against using in production.
* Mention `pub outdated --mode=null-safety`.
* Mention migrating a clean code repository.
* Add more text about committing along the way.
* Make clear that the tool provides no revert capability.

Change-Id: I7914dc904a93046ce2508c51cae158ac39e889af
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/150822
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Mike Fairhurst <mfairhurst@google.com>

pkg/nnbd_migration/README.md

@@ -1,18 +1,18 @@
-# Null Safety Migration Tooling
+# Null safety migration tooling
 
 **Note**:
 
   * The null safety migration tooling is in an early state and may have bugs and
     other issues.
   * As null safety is still in preview, we recommend only doing trial
-    migrations. The final migration of apps and packages, should not be done
+    migrations. The final migration of apps and packages should not be done
     until the feature is more complete.
   * For best results, use SDK version 2.9.0-10.0.dev or higher.
 
-## How Migration Works
+## How migration works
 
-The migration uses a _new interactive algorithm_ designed specifically for Dart
+The migration uses a _new interactive algorithm_ designed specifically for [Dart
-null safety.
+null safety](https://dart.dev/null-safety).
 
 Typical code migration tools are designed to be run once, handle most cases, and
 let the developer do manual cleanup on the result. This does **not work well**
@@ -42,12 +42,12 @@ Adding a small number of these hints will have a huge impact on migration
 quality.
 
 The high level workflow of the tool is therefore driven through an interactive
-web UI. After running `dart migrate`, open the resulting URL in a browser. Scan
+web UI. After starting the tool with `dart migrate`, open the presented URL in a
-through the changes, use the "nullability trace" feature to find the best place
+browser. Scan through the changes, use the "nullability trace" feature to find
-to add a nullability hint (adding a hint in the best place can prevent dozens of
+the best place to add a nullability hint (adding a hint in the best place can
-types from being made nullable). Rerun the migration and repeat, committing the
+prevent dozens of types from being made nullable). Rerun the migration and
-hints as you go. When the output is correct and acceptable, apply the migration
+repeat, committing the hints as you go. When the output is correct and
-and publish your null-safe code!
+acceptable, apply the migration.
 
 For example,
 
@@ -92,12 +92,25 @@ change large numbers of types for you at once without also accidentally changing
 your intent has been lost. A long migration effort (such as one on a large
 project) can be done incrementally, by committing these hints over time.
 
+<!-- TODO(srawlins): We should explain (or point to explanation of) "migrated"
+code. I don't see any documents pointing out how null safety is enabled via
+pubspec.yaml, or library-by-library comments. -->
+
 ## Migrating a package
 
 1. Select a package to work on, and open a command terminal in the top-level of
    the package directory.
 2. Run `pub get` in order to make available all dependencies of the package.
-3. Run the migration tool from the top-level of the package directory:
+3. It is best to migrate a package to null safety _after_ the package's
+   dependencies have migrated to null safety. Run
+   `pub outdated --mode=null-safety` to learn the migration status of the
+   package's dependencies. See the
+   [pub outdated documentation](https://dart.dev/tools/pub/cmd/pub-outdated)
+   for more information.
+4. It is best to migrate a package starting from a clean code repository state
+   (`git status`, for example), in case you must revert the migration. Ensure
+   there are no pending changes in the package's code repository.
+5. Run the migration tool from the top-level of the package directory:
 
    ```
    dart migrate
@@ -117,10 +130,16 @@ browser to view, analyze, and improve the proposed null-safe migration.
     1. Use the "trace view" in the bottom right to find the root cause
     2. Either click on an "add hint" button to correct it at the root, or open
        your editor and make the change manually.
-        1. Some changes are as simple as adding a `/*!*/` hint on a type. The
+        * Some changes are as simple as adding a `/*!*/` hint on a type. The
-           tool has buttons to do this for you.
+          tool has buttons to do this for you.
-        2. Others may require larger refactors. These changes can be made in
+        * Others may require larger refactors. These changes can be made in
-           your editor, and may often be committed and published immediately.
+          your editor.
+        * Changes may even be committed to source code management before finally
+         _applying_ the migration. In this way, a migration of a large package
+         can be carried out over multiple sessions, or between multiple
+         engineers. Committing hints and other adjustments along the way helps
+         to separate the concerns of describing user intent vs committing to the
+         migration result.
     3. Periodically rerun the migration and repeat.
 6. Once you are satisfied with the proposed migration:
     1. Save your work using git or other means. Applying the migration will
@@ -134,12 +153,16 @@ browser to view, analyze, and improve the proposed null-safe migration.
        interface.
     3. Tip: leaving the web UI open may help you if you later have test failures
        or analysis errors.
-7. Rerun `pub get` and test your package.
+7. Rerun `pub get`, then analyze and test your package.
-    1. If a test fails, you may still use the preview to help you figure out
+    1. If there are new static analysis issues, or if a test fails, you may
-       what went wrong.
+       still use the preview to help you figure out what went wrong.
     2. If large changes are required, revert the migration, and go back to step
-       one.
+       one. The tool does not provide any revert capability; this must be done
-8. Commit and/or publish your migrated null-safe code.
+       via source code management (for example, `git checkout`).
+
+<!-- TODO(srawlins): direct the user to publish, only after null safety leaves
+tech preview. See the big note at https://dart.dev/null-safety.
+8. Commit and/or publish your migrated null-safe code. -->
 
 ## Providing feedback