Flutter shouldn't throw exception when adb tool is not installed #380

$ flutter start
Downloading Sky Snapshot from the cloud, one moment please...
Downloading Sky Shell from the cloud, one moment please...
ProcessException: No such file or directory
  Command: adb devices -l
severe: 
Exception:
dart:io                                                             Process.runSync
package:flutter_tools/src/process.dart 82                           _runWithLoggingSync
package:flutter_tools/src/process.dart 71                           runSync
package:flutter_tools/src/device.dart 508                           AndroidDevice.getAttachedDevices
package:flutter_tools/src/device.dart 881                           DeviceStore.DeviceStore.forConfigs
package:flutter_tools/src/commands/flutter_command.dart 36          FlutterCommand.connectToDevices
package:flutter_tools/src/commands/flutter_command.dart 41          FlutterCommand.downloadApplicationPackagesAndConnectToDevices.<async>
dart:async                                                          _Completer.completeError
package:flutter_tools/src/commands/flutter_command.dart 44          FlutterCommand.downloadApplicationPackagesAndConnectToDevices.<async>
dart:async                                                          _Future.then
package:flutter_tools/src/commands/flutter_command.dart 40          FlutterCommand.downloadApplicationPackagesAndConnectToDevices.<async>
dart:async                                                          Future.Future.microtask
package:flutter_tools/src/commands/flutter_command.dart             FlutterCommand.downloadApplicationPackagesAndConnectToDevices
package:flutter_tools/src/commands/start.dart 44                    StartCommand.runInProject.<async>
dart:async                                                          Future.Future.microtask
package:flutter_tools/src/commands/start.dart                       StartCommand.runInProject
package:flutter_tools/src/commands/flutter_command.dart 59          FlutterCommand.run.<async>
dart:async                                                          Future.Future.microtask
package:flutter_tools/src/commands/flutter_command.dart             FlutterCommand.run
package:args/command_runner.dart 178                                CommandRunner.runCommand.<fn>
dart:async                                                          Future.Future.sync
package:args/command_runner.dart 131                                CommandRunner.runCommand
package:flutter_tools/src/commands/flutter_command_runner.dart 141  FlutterCommandRunner.runCommand
package:args/command_runner.dart 105                                CommandRunner.run.<fn>
dart:async                                                          Future.Future.sync
package:args/command_runner.dart 105                                CommandRunner.run
package:flutter_tools/executable.dart 67                            main.<async>.<fn>.<async>
package:stack_trace                                                 Chain.capture
package:flutter_tools/executable.dart 66                            main.<async>