[script/tool] Use 'dart pub' instead of deprecated 'pub'

[mrhota]

Because I'm new to dart and flutter, I never had a pub binary installed. Running dart run script/tool/lib/src/main.dart test crashed on the plugin_platform_interface tests because that package doesn't qualify as a flutter package. The log message at the point of failure hinted at the cause:

RUNNING plugin_platform_interface tests...
Running command: "pub get" in /Users/aj/dev/flutter_plugins/packages/plugin_platform_interface
Unhandled exception:
ProcessException: No such file or directory
  Command: pub get
#0      _ProcessImpl._start (dart:io-patch/process_patch.dart:390:33)
#1      Process.start (dart:io-patch/process_patch.dart:36:20)
#2      ProcessRunner.runAndStream (package:flutter_plugin_tools/src/common.dart:540:49)
#3      TestCommand.run (package:flutter_plugin_tools/src/test_command.dart:68:40)
<asynchronous suspension>
#4      CommandRunner.runCommand (package:args/command_runner.dart:196:13)
<asynchronous suspension>
#5      main.<anonymous closure> (package:flutter_plugin_tools/src/main.dart)
<asynchronous suspension>

List which issues are fixed by this PR. You must list at least one issue.
I couldn't find one. This is a test/tool infrastructure fix.

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format. See plugin_tool format)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[mrhota]

I don't know why the submit-queue check failed here.

[stuartmorgan-g]

LGTM, thanks!

The submit-queue redness looks like Cirrus didn't report status back correctly for the last commit on master.