docs: Improves READMEs styles (#919)

packages/firebase_admob/README.md

@@ -28,13 +28,13 @@ and setting up your App ID.
 ## Initializing the plugin
 The AdMob plugin must be initialized with an AdMob App ID.
 
-```
+```dart
 FirebaseAdMob.instance.initialize(appId: appId);
 ```
 *Note Android*:
 Starting in version 17.0.0, if you are an AdMob publisher you are now required to add your AdMob app ID in your **AndroidManifest.xml** file. Once you find your AdMob app ID in the AdMob UI, add it to your manifest adding the following tag:
 
-```
+```xml
 <manifest>
     <application>
         <!-- TODO: Replace with your real AdMob app ID -->
@@ -90,7 +90,7 @@ InterstitialAd myInterstitial = InterstitialAd(
 ```
 
 Ads must be loaded before they're shown.
-```
+```dart
 myBanner
   // typically this happens well before the ad is shown
   ..load()
@@ -102,7 +102,7 @@ myBanner
   );
 ```
 
-```
+```dart
 myInterstitial
   ..load()
   ..show(
@@ -124,7 +124,7 @@ listener can be used to detect when the ad has actually finished loading
 Unlike banners and interstitials, rewarded video ads are loaded one at a time
 via a singleton object, `RewardedVideoAd.instance`. Its `load` method takes an
 AdMob ad unit ID and an instance of `MobileAdTargetingInfo`:
-```
+```dart
 RewardedVideoAd.instance.load(myAdMobAdUnitId, targetingInfo);
 ```
 
@@ -135,13 +135,13 @@ function will be invoked whenever one of the events in the `RewardedVideAdEvent`
 enum occurs. After a rewarded video ad loads, for example, the
 `RewardedVideoAdEvent.loaded` is sent. Any time after that, apps can show the ad
 by calling `show`:
-```
+```dart
 RewardedVideoAd.instance.show();
 ```
 
 When the AdMob SDK decides it's time to grant an in-app reward, it does so via
 the `RewardedVideoAdEvent.rewarded` event:
-```
+```dart
 RewardedVideoAd.instance.listener =
     (RewardedVideoAdEvent event, [String rewardType, int rewardAmount]) {
   if (event == RewardedVideoAdEvent.rewarded) {

packages/firebase_ml_vision/README.md

@@ -15,7 +15,7 @@ To use this plugin, add `firebase_ml_vision` as a [dependency in your pubspec.ya
 ### Android
 Optional but recommended: If you use the on-device API, configure your app to automatically download the ML model to the device after your app is installed from the Play Store. To do so, add the following declaration to your app's AndroidManifest.xml file:
 
-```manifest
+```xml
 <application ...>
   ...
   <meta-data

packages/firebase_remote_config/README.md

@@ -18,7 +18,7 @@ To use the firebase_remote_config plugin, follow the [plugin installation instru
 Enable the Google services by configuring the Gradle scripts as such.
 
 1. Add the classpath to the `[project]/android/build.gradle` file.
-```
+```gradle
 dependencies {
   // Example existing classpath
   classpath 'com.android.tools.build:gradle:3.2.1'
@@ -28,7 +28,7 @@ dependencies {
 ```
 
 2. Add the apply plugin to the `[project]/android/app/build.gradle` file.
-```
+```gradle
 // ADD THIS AT THE BOTTOM
 apply plugin: 'com.google.gms.google-services'
 ```
@@ -46,17 +46,17 @@ Otherwise you will not be able to use Firebase Remote Config.
 ### Use the plugin
 
 Add the following imports to your Dart code:
-```
+```dart
 import 'package:firebase_remote_config/firebase_remote_config.dart';
 ```
 
 Initialize `RemoteConfig`:
-```
+```dart
 final RemoteConfig remoteConfig = await RemoteConfig.instance;
 ```
 
 You can now use the Firebase `remoteConfig` to fetch remote configurations in your Dart code, e.g.
-```
+```dart
 final defaults = <String, dynamic>{'welcome': 'default welcome'};
 await remoteConfig.setDefaults(defaults);
 

packages/google_sign_in/README.md

@@ -26,7 +26,7 @@ enable the [Google People API](https://developers.google.com/people/).
 4. A dialog will show up and ask you to select the targets, select the `Runner` target.
 5. Then add the `CFBundleURLTypes` attributes below into the `[my_project]/ios/Runner/Info.plist` file.
 
-```
+```xml
 <!-- Put me in the [my_project]/ios/Runner/Info.plist file -->
 <!-- Google Sign-in Section -->
 <key>CFBundleURLTypes</key>
@@ -53,7 +53,7 @@ To use this plugin, follow the [plugin installation instructions](https://pub.da
 ### Use the plugin
 Add the following import to your Dart code:
 
-```
+```dart
 import 'package:google_sign_in/google_sign_in.dart';
 ```
 
@@ -71,7 +71,7 @@ GoogleSignIn _googleSignIn = GoogleSignIn(
 
 You can now use the `GoogleSignIn` class to authenticate in your Dart code, e.g.
 
-```
+```dart
 Future<void> _handleSignIn() async {
   try {
     await _googleSignIn.signIn();

packages/local_auth/README.md

@@ -116,7 +116,7 @@ try {
 Note that this plugin works with both TouchID and FaceID. However, to use the latter,
 you need to also add:
 
-```
+```xml
 <key>NSFaceIDUsageDescription</key>
 <string>Why is my app authenticating using face id?</string>
 ```
@@ -130,7 +130,7 @@ app has not been updated to use TouchID.
 Update your project's `AndroidManifest.xml` file to include the
 `USE_FINGERPRINT` permissions:
 
-```
+```xml
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
           package="com.example.app">
   <uses-permission android:name="android.permission.USE_FINGERPRINT"/>

packages/shared_preferences/README.md

@@ -41,7 +41,7 @@ _incrementCounter() async {
 
 You can populate `SharedPreferences` with initial values in your tests by running this code:
 
-```
+```dart
 const MethodChannel('plugins.flutter.io/shared_preferences')
   .setMockMethodCallHandler((MethodCall methodCall) async {
     if (methodCall.method == 'getAll') {