Revert "Roll Dart SDK from fe12b0536f42 to a188781c9fc8 (1 revision)"

[zanderso]

Reverts #22379

pub is crashing on the engine -> framework roll:

Running "flutter pub get" in project_root...                    
pubspec.yaml has no lower-bound SDK constraint.

You should edit pubspec.yaml to contain an SDK constraint:



environment:

  sdk: '>=2.10.0 <3.0.0'



See https://dart.dev/go/sdk-constraint

---- Log transcript ----

FINE: Pub 2.12.0-28.0.dev

ERR : pubspec.yaml has no lower-bound SDK constraint.
    | You should edit pubspec.yaml to contain an SDK constraint:
    | 
    | environment:
    |   sdk: '>=2.10.0 <3.0.0'
    | 






    | See https://dart.dev/go/sdk-constraint
FINE: Exception type: DataException
FINE: package:pub/src/entrypoint.dart 848:5       _checkSdkConstraintIsDefined
    | package:pub/src/entrypoint.dart 230:5       Entrypoint.acquireDependencies
    | package:pub/src/command/get.dart 43:23      GetCommand.runProtected
    | dart:async                                  new Future.sync
    | package:pub/src/utils.dart 113:12           captureErrors.wrappedCallback
    | package:stack_trace                         Chain.capture
    | package:pub/src/utils.dart 126:11           captureErrors
    | package:pub/src/command.dart 146:13         PubCommand.run
    | package:args/command_runner.dart 197:27     CommandRunner.runCommand
    | package:pub/src/command_runner.dart 150:24  PubCommandRunner.runCommand
    | ===== asynchronous gap ===========================
    | dart:async                                  Future.catchError
    | package:pub/src/utils.dart 113:52           captureErrors.wrappedCallback
    | package:stack_trace                         Chain.capture
    | package:pub/src/utils.dart 126:11           captureErrors
    | package:pub/src/command.dart 146:13         PubCommand.run
    | package:args/command_runner.dart 197:27     CommandRunner.runCommand
    | package:pub/src/command_runner.dart 150:24  PubCommandRunner.runCommand
---- End log transcript ----