Update null safety example for 2.12 (#90)

* Update readme and pubspec

* Remove unneeded IDE configs

null_safety/calculate_lix/README.md

@@ -2,23 +2,12 @@
 
 This is a small code example of an app that calculates the 'lix' readability
 index for a text file. The implementation uses the Dart null safety feature,
-and is meant to demonstrate how this feature works in a practical example,
-as well as serve as a demonstration of how to configure and run code with null
-safety at its current tech preview stage.
+and is meant to demonstrate how this feature works in a practical example.
 
-## Running the example code
+## Supported SDK versions
 
-The code works only with preview builds of null safety from the `beta`
-channel. If you're using Flutter, you can run:
-
-```cmd
-flutter channel beta
-flutter upgrade
-```
-
-to switch to this channel; alternatively, you may download and install the
-latest null safety preview build of Dart from the [Dart SDK
-archive](https://dart.dev/tools/sdk/archive#beta-channel).
+The example requires Dart 2.12 or later. If you're using Flutter, you need
+Flutter 2.0 or later.
 
 ### Running from the terminal / command prompt
 
@@ -30,49 +19,6 @@ $ dart pub get
 $ dart run bin/main.dart text/lorem-ipsum.txt
 ```
 
-### Running from VSCode
-
-This example contains a launch configuration for VSCode that runs
-`bin/main.dart`, so to run the sample in
-VSCode:
-
-  1. If you're using a separate copy of Dart, rather than the Flutter `dev`
-     channel, edit your VSCode configuration to point to one additional Dart
-     SDK, the preview SDK we just downloaded. See [details
-     here](https://dartcode.org/docs/quickly-switching-between-sdk-versions/)
-     for what values to put in **View > Command Palette > Open Settings
-     (JSON)**.
-
-  1. Invoke **File > Open**, and select the `calculate_lix` folder.
-
-  1. Tell VSCode to use the preview Dart SDK: Open `bin/main.dart` and then
-     locate the 'Dart: \<version number\>' selector in the status bar at the
-     bottom, and select `Dart: 2.12.0-xxx.0.beta` (where `xxx` is the latest
-     version available.)
-
-  1. Select **Run > Run** and the project should run and print a message in the Debug
-     Console.
-
-
-### Running from Android Studio
-
-  1. Start Android Studio.
-
-  1. Select Open Project, and select the `calculate_lix` folder.
-
-  1. Open the file `bin/main.dart` in the code editor.
-
-  1. Select 'Open Dart Settings' in the top banner.
-
-  1. Select both 'Enable Dart support' checkmarks at the top and bottom of the dialog.
-
-  1. Under Dart SDK specify the path to the Dart preview SDK
-     (`2.12.0-xxx.0.beta`, where `xxx` is the latest version available). Click
-     OK.
-
-  1. Select Run > Run and the project should run and print a message in the Run
-     pane.
-
 ## Next steps
 
 Once you have the code running, here some suggestions for things to try:

null_safety/calculate_lix/pubspec.lock

@@ -7,6 +7,6 @@ packages:
       name: collection
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.15.0-nullsafety.5"
+    version: "1.15.0"
 sdks:
-  dart: ">=2.12.0-29 <3.0.0"
+  dart: ">=2.12.0 <3.0.0"

null_safety/calculate_lix/pubspec.yaml

@@ -1,12 +1,12 @@
 name: calculate_lix
 description: A command-line application demonstrating use of null safety.
-version: 1.2.0
+version: 1.3.0
 
 # This example isn't intended for publishing on pub.dev.
 publish_to: none
 
 environment:
-  sdk: '>=2.12.0-29 <3.0.0'
+  sdk: '>=2.12.0 <3.0.0'
 
 dependencies:
-  collection: ^1.15.0-nullsafety
+  collection: ^1.15.0