Fixes a common crash in getAdbDevices when adb executable isn't found. (#22676)

Apparently, this accounts for 13% of our crashes in Beta.

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

@@ -533,7 +533,13 @@ List<AndroidDevice> getAdbDevices() {
   final String adbPath = getAdbPath(androidSdk);
   if (adbPath == null)
     return <AndroidDevice>[];
-  final String text = runSync(<String>[adbPath, 'devices', '-l']);
+  String text;
+  try {
+    text = runSync(<String>[adbPath, 'devices', '-l']);
+  } on ArgumentError catch (exception) {
+    throwToolExit('Unable to run "adb", check your Android SDK installation and '
+      'ANDROID_HOME environment variable: ${exception.message}');
+  }
   final List<AndroidDevice> devices = <AndroidDevice>[];
   parseADBDeviceOutput(text, devices: devices);
   return devices;

packages/flutter_tools/test/android/android_device_test.dart

@@ -4,13 +4,18 @@
 
 import 'dart:async';
 
+import 'package:file/memory.dart';
 import 'package:flutter_tools/src/android/android_device.dart';
+import 'package:flutter_tools/src/android/android_sdk.dart';
+import 'package:flutter_tools/src/base/config.dart';
+import 'package:flutter_tools/src/base/file_system.dart';
 import 'package:flutter_tools/src/base/io.dart';
 import 'package:mockito/mockito.dart';
 import 'package:process/process.dart';
 
 import '../src/common.dart';
 import '../src/context.dart';
+import '../src/mocks.dart';
 
 void main() {
   group('android_device', () {
@@ -22,6 +27,18 @@ void main() {
   });
 
   group('getAdbDevices', () {
+    testUsingContext('throws on missing adb path', () {
+      final Directory sdkDir = MockAndroidSdk.createSdkDirectory();
+      Config.instance.setValue('android-sdk', sdkDir.path);
+
+      final File adbExe = fs.file(getAdbPath(androidSdk));
+      adbExe.deleteSync();
+      expect(() => getAdbDevices(), throwsToolExit(message: RegExp('Unable to run "adb".*${adbExe.path}')));
+    }, overrides: <Type, Generator>{
+      AndroidSdk: () => MockAndroidSdk(),
+      FileSystem: () => MemoryFileSystem(),
+    });
+
     testUsingContext('physical devices', () {
       final List<AndroidDevice> devices = <AndroidDevice>[];
       parseADBDeviceOutput('''

packages/flutter_tools/test/src/common.dart

@@ -84,7 +84,7 @@ void updateFileModificationTime(String path,
 }
 
 /// Matcher for functions that throw [ToolExit].
-Matcher throwsToolExit({int exitCode, String message}) {
+Matcher throwsToolExit({int exitCode, Pattern message}) {
   Matcher matcher = isToolExit;
   if (exitCode != null)
     matcher = allOf(matcher, (ToolExit e) => e.exitCode == exitCode);