display ANDROID_HOME env var in flutter doctor (#6558)

packages/flutter_tools/lib/src/android/android_sdk.dart

diff --git a/packages/flutter_tools/lib/src/android/android_sdk.dart b/packages/flutter_tools/lib/src/android/android_sdk.dart
index 65e3edc..ffe5d90 100644
--- a/packages/flutter_tools/lib/src/android/android_sdk.dart
+++ b/packages/flutter_tools/lib/src/android/android_sdk.dart
@@ -11,6 +11,8 @@
 import '../base/os.dart';
 import '../globals.dart';
 
+const String kAndroidHome = 'ANDROID_HOME';
+
 // Android SDK layout:
 
 // $ANDROID_HOME/platform-tools/adb
@@ -58,8 +60,8 @@
 
   static AndroidSdk locateAndroidSdk() {
     String androidHomeDir;
-    if (Platform.environment.containsKey('ANDROID_HOME')) {
-      androidHomeDir = Platform.environment['ANDROID_HOME'];
+    if (Platform.environment.containsKey(kAndroidHome)) {
+      androidHomeDir = Platform.environment[kAndroidHome];
     } else if (Platform.isLinux) {
       if (homeDirPath != null)
         androidHomeDir = '$homeDirPath/Android/Sdk';

packages/flutter_tools/lib/src/android/android_workflow.dart

diff --git a/packages/flutter_tools/lib/src/android/android_workflow.dart b/packages/flutter_tools/lib/src/android/android_workflow.dart
index 472f6c4..05bc39a 100644
--- a/packages/flutter_tools/lib/src/android/android_workflow.dart
+++ b/packages/flutter_tools/lib/src/android/android_workflow.dart
@@ -29,10 +29,18 @@
     String sdkVersionText;
 
     if (androidSdk == null) {
-      messages.add(new ValidationMessage.error(
-        'Android Studio / Android SDK not found. Download from https://developer.android.com/sdk/\n'
-        '(or visit https://flutter.io/setup/#android-setup for detailed instructions).'
-      ));
+      if (Platform.environment.containsKey(kAndroidHome)) {
+        String androidHomeDir = Platform.environment[kAndroidHome];
+        messages.add(new ValidationMessage.error(
+          '$kAndroidHome = $androidHomeDir\n'
+          'but Android Studio / Android SDK not found at this location.'
+        ));
+      } else {
+        messages.add(new ValidationMessage.error(
+          'Android Studio / Android SDK not found. Download from https://developer.android.com/sdk/\n'
+          '(or visit https://flutter.io/setup/#android-setup for detailed instructions).'
+        ));
+      }
     } else {
       type = ValidationType.partial;
 
@@ -47,6 +55,11 @@
         ));
       }
 
+      if (Platform.environment.containsKey(kAndroidHome)) {
+        String androidHomeDir = Platform.environment[kAndroidHome];
+        messages.add(new ValidationMessage('$kAndroidHome = $androidHomeDir'));
+      }
+
       List<String> validationResult = androidSdk.validateSdkWellFormed();
 
       if (validationResult.isEmpty) {