我正在做Firebase for Flutter练习。我完全按照说明进行操作（在firebase上设置，导入conif文档，修改pubspec.yaml等）。
但是，当我在iPhone模拟器上启动该应用程序时，cocoapods尝试安装cloud_firestore时出现错误。
我在此处复制执行flutter run -v的信息
[ +298 ms] Building bundle
[ ] Writing asset files to build/flutter_assets
[ +57 ms] Wrote build/flutter_assets
[ +1 ms] executing: /usr/bin/xcrun simctl get_app_container C8322200-306D-4478-8969-B5124B3FA08F com.example.flutterFirebase
[ ] executing: /usr/bin/killall Runner
[ +141 ms] executing: [ios/] /usr/bin/xcodebuild -list
[ +713 ms] Exit code 0 from: /usr/bin/xcodebuild -list
[ ] Information about project "Runner":
Targets:
Runner
Build Configurations:
Debug
Release
If no build configuration is specified and -scheme is not passed then "Release" is used.
Schemes:
Runner
[ +2 ms] Trying to resolve native pub services.
[ +2 ms] Looking for YAML at 'pubspec.yaml'
[ +1 ms] No services specified in the manifest
[ ] Found 0 service definition(s).
[ +1 ms] Copying service frameworks to '/Users/adriendulong/Documents/Dev/flutter_firebase/ios/Frameworks'.
[ ] Creating service definitions manifest at 'ios/ServiceDefinitions.json'
[ +20 ms] executing: pod --version
[ +525 ms] 1.5.3
[ +3 ms] Running pod install...
[ +811 ms] Running pod install... (completed)
[ ] CocoaPods' output:
↳
[ ] Preparing
Analyzing dependencies
Inspecting targets to integrate
Using `ARCHS` setting to build architectures of target `Pods-Runner`: (``)
Fetching external sources
-> Fetching podspec for `Flutter` from `.symlinks/flutter/ios`
-> Fetching podspec for `cloud_firestore` from `.symlinks/plugins/cloud_firestore/ios`
-> Fetching podspec for `firebase_core` from `.symlinks/plugins/firebase_core/ios`
Resolving dependencies of `Podfile`
[!] Unable to find a specification for `Firebase/Database` depended upon by `cloud_firestore`
/usr/local/lib/ruby/gems/2.4.0/gems/cocoapods-1.5.3/lib/cocoapods/resolver.rb:402:in `find_cached_set'
/usr/local/lib/ruby/gems/2.4.0/gems/cocoapods-1.5.3/lib/cocoapods/resolver.rb:372:in `specifications_for_dependency'
/usr/local/lib/ruby/gems/2.4.0/gems/cocoapods-1.5.3/lib/cocoapods/resolver.rb:177:in `search_for'
/usr/local/lib/ruby/gems/2.4.0/gems/cocoapods-1.5.3/lib/cocoapods/resolver.rb:288:in `block in sort_dependencies'
/usr/local/lib/ruby/gems/2.4.0/gems/cocoapods-1.5.3/lib/cocoapods/resolver.rb:281:in `each'
/usr/local/lib/ruby/gems/2.4.0/gems/cocoapods-1.5.3/lib/cocoapods/resolver.rb:281:in `sort_by'
/usr/local/lib/ruby/gems/2.4.0/gems/cocoapods-1.5.3/lib/cocoapods/resolver.rb:281:in `sort_dependencies'
/usr/local/lib/ruby/gems/2.4.0/gems/molinillo-0.6.5/lib/molinillo/delegates/specification_provider.rb:53:in `block in sort_dependencies'
/usr/local/lib/ruby/gems/2.4.0/gems/molinillo-0.6.5/lib/molinillo/delegates/specification_provider.rb:70:in `with_no_such_dependency_error_handling'
/usr/local/lib/ruby/gems/2.4.0/gems/molinillo-0.6.5/lib/molinillo/delegates/specification_provider.rb:52:in `sort_dependencies'
/usr/local/lib/ruby/gems/2.4.0/gems/molinillo-0.6.5/lib/molinillo/resolution.rb:754:in `push_state_for_requirements'
/usr/local/lib/ruby/gems/2.4.0/gems/molinillo-0.6.5/lib/molinillo/resolution.rb:746:in `require_nested_dependencies_for'
/usr/local/lib/ruby/gems/2.4.0/gems/molinillo-0.6.5/lib/molinillo/resolution.rb:729:in `activate_new_spec'
/usr/local/lib/ruby/gems/2.4.0/gems/molinillo-0.6.5/lib/molinillo/resolution.rb:686:in `attempt_to_activate'
/usr/local/lib/ruby/gems/2.4.0/gems/molinillo-0.6.5/lib/molinillo/resolution.rb:254:in `process_topmost_state'
/usr/local/lib/ruby/gems/2.4.0/gems/molinillo-0.6.5/lib/molinillo/resolution.rb:182:in `resolve'
/usr/local/lib/ruby/gems/2.4.0/gems/molinillo-0.6.5/lib/molinillo/resolver.rb:43:in `resolve'
/usr/local/lib/ruby/gems/2.4.0/gems/cocoapods-1.5.3/lib/cocoapods/resolver.rb:123:in `resolve'
/usr/local/lib/ruby/gems/2.4.0/gems/cocoapods-1.5.3/lib/cocoapods/installer/analyzer.rb:781:in `block in resolve_dependencies'
/usr/local/lib/ruby/gems/2.4.0/gems/cocoapods-1.5.3/lib/cocoapods/user_interface.rb:64:in `section'
/usr/local/lib/ruby/gems/2.4.0/gems/cocoapods-1.5.3/lib/cocoapods/installer/analyzer.rb:779:in `resolve_dependencies'
/usr/local/lib/ruby/gems/2.4.0/gems/cocoapods-1.5.3/lib/cocoapods/installer/analyzer.rb:88:in `analyze'
/usr/local/lib/ruby/gems/2.4.0/gems/cocoapods-1.5.3/lib/cocoapods/installer.rb:243:in `analyze'
/usr/local/lib/ruby/gems/2.4.0/gems/cocoapods-1.5.3/lib/cocoapods/installer.rb:154:in `block in resolve_dependencies'
/usr/local/lib/ruby/gems/2.4.0/gems/cocoapods-1.5.3/lib/cocoapods/user_interface.rb:64:in `section'
/usr/local/lib/ruby/gems/2.4.0/gems/cocoapods-1.5.3/lib/cocoapods/installer.rb:153:in `resolve_dependencies'
/usr/local/lib/ruby/gems/2.4.0/gems/cocoapods-1.5.3/lib/cocoapods/installer.rb:116:in `install!'
/usr/local/lib/ruby/gems/2.4.0/gems/cocoapods-1.5.3/lib/cocoapods/command/install.rb:41:in `run'
/usr/local/lib/ruby/gems/2.4.0/gems/claide-1.0.2/lib/claide/command.rb:334:in `run'
/usr/local/lib/ruby/gems/2.4.0/gems/cocoapods-1.5.3/lib/cocoapods/command.rb:52:in `run'
/usr/local/lib/ruby/gems/2.4.0/gems/cocoapods-1.5.3/bin/pod:55:in `<top (required)>'
/usr/local/bin/pod:22:in `load'
/usr/local/bin/pod:22:in `<main>'
[ +4 ms] Error running pod install
[ +1 ms] Error launching application on iPhone X.
[ +16 ms] "flutter run" took 6 280ms.
#0 throwToolExit (package:flutter_tools/src/base/common.dart:26:3)
#1 RunCommand.runCommand (package:flutter_tools/src/commands/run.dart:417:7)
<asynchronous suspension>
#2 FlutterCommand.verifyThenRunCommand (package:flutter_tools/src/runner/flutter_command.dart:356:18)
#3 _asyncThenWrapperHelper.<anonymous closure> (dart:async/runtime/libasync_patch.dart:77:64)
#4 _rootRunUnary (dart:async/zone.dart:1132:38)
#5 _CustomZone.runUnary (dart:async/zone.dart:1029:19)
#6 _FutureListener.handleValue (dart:async/future_impl.dart:129:18)
#7 Future._propagateToListeners.handleValueCallback (dart:async/future_impl.dart:642:45)
#8 Future._propagateToListeners (dart:async/future_impl.dart:671:32)
#9 Future._complete (dart:async/future_impl.dart:476:7)
#10 _SyncCompleter.complete (dart:async/future_impl.dart:51:12)
#11 _AsyncAwaitCompleter.complete (dart:async/runtime/libasync_patch.dart:28:18)
#12 _completeOnAsyncReturn (dart:async/runtime/libasync_patch.dart:295:13)
#13 RunCommand.usageValues (package:flutter_tools/src/commands/run.dart)
#14 _asyncThenWrapperHelper.<anonymous closure> (dart:async/runtime/libasync_patch.dart:77:64)
#15 _rootRunUnary (dart:async/zone.dart:1132:38)
#16 _CustomZone.runUnary (dart:async/zone.dart:1029:19)
#17 _FutureListener.handleValue (dart:async/future_impl.dart:129:18)
#18 Future._propagateToListeners.handleValueCallback (dart:async/future_impl.dart:642:45)
#19 Future._propagateToListeners (dart:async/future_impl.dart:671:32)
#20 Future._complete (dart:async/future_impl.dart:476:7)
#21 _SyncCompleter.complete (dart:async/future_impl.dart:51:12)
#22 _AsyncAwaitCompleter.complete.<anonymous closure> (dart:async/runtime/libasync_patch.dart:33:20)
#23 _rootRun (dart:async/zone.dart:1124:13)
#24 _CustomZone.run (dart:async/zone.dart:1021:19)
#25 _CustomZone.bindCallback.<anonymous closure> (dart:async/zone.dart:947:23)
#26 _microtaskLoop (dart:async/schedule_microtask.dart:41:21)
#27 _startMicrotaskLoop (dart:async/schedule_microtask.dart:50:5)
#28 _runPendingImmediateCallback (dart:isolate/runtime/libisolate_patch.dart:115:13)
#29 _RawReceivePortImpl._handleMessage (dart:isolate/runtime/libisolate_patch.dart:172:5)
谢谢