Race condition initializing AnalyzerResolvers

[natebosch]

When using build_runner there is only ever a single instance of AnalyzerResolvers, however when using build_test there can be multiple instances constructed. Potentially indifferent isolates.

During initialization this class touches the filesystem and initializing more than one has race conditions deleting a file.

  FileSystemException: Cannot delete file, path = '.dart_tool/build_resolvers/sdk.sum.deps' (OS Error: No such file or directory, errno = 2)
  dart:io                                               FileSystemEntity.delete
  package:build_resolvers/src/resolver.dart 374:22      _defaultSdkSummaryGenerator

https://github.com/dart-lang/build/blob/cc629116eef498015d9991bc8760f0b3e73f03e3/build_resolvers/lib/src/resolver.dart#L372-L374