Add a diagnostic message in analyze.dart

[LongCatIsLooong]

Description

I ran into an analyzer failure in
https://github.com/flutter/flutter/pull/37719/checks?check_run_id=191556624 because I wrote
import 'package:flutter/src/widgets/basic.dart';

the output wasn't the most helpful:

dart --enable-asserts ./dev/bots/analyze.dart
⏩ RUNNING: cd examples/hello_world; ../../bin/flutter inject-plugins
🕐 ELAPSED TIME: 0:00:01.003618 for ../../bin/flutter inject-plugins in examples/hello_world: 
Unhandled exception:
NoSuchMethodError: The getter 'iterator' was called on null.
Receiver: null
Tried calling: iterator
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:50:5)
#1      _deepSearch (file:///tmp/flutter%20sdk/dev/bots/analyze.dart:459:20)
#2      _deepSearch (file:///tmp/flutter%20sdk/dev/bots/analyze.dart:464:28)
#3      _verifyNoBadImportsInFlutter (file:///tmp/flutter%20sdk/dev/bots/analyze.dart:393:31)
<asynchronous suspension>
#4      main (file:///tmp/flutter%20sdk/dev/bots/analyze.dart:37:9)
<asynchronous suspension>
#5      _startIsolate.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:303:32)
#6      _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:172:12)

Exit status: 255

Now it should say:

⏩ RUNNING: cd ../integration_tests/ui; ../../../bin/flutter inject-plugins
🕐 ELAPSED TIME: 0:00:00.582985 for ../../../bin/flutter inject-plugins in ../integration_tests/ui:
⏩ RUNNING: cd ../../examples/hello_world; ../../bin/flutter inject-plugins
🕐 ELAPSED TIME: 0:00:00.569801 for ../../bin/flutter inject-plugins in ../../examples/hello_world:
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
An error was detected when looking at import dependencies within the Flutter package:

cupertino imported package:flutter/src/widgets/basic.dart which is not one of the valid exports { animation.dart, cupertino.dart, foundation.dart, gestures.dart, material.dart, painting.dart, physics.dart, rendering.dart, scheduler.dart, semantics.dart, services.dart, widgets.dart }.                                  
Consider changing src/widgets/basic.dart to one of them.
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

Tests

Not sure if tests are needed?

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I signed the CLA.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require Flutter developers to manually update their apps to accommodate your change?

• No, this is not a breaking change.

[fluttergithubbot]

It looks like this pull request may not have tests. Please make sure to add tests before merging. While there are exceptions to this rule, if this patch modifies code it is probably not an exception.

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[goderbauer]

LGTM