title: Error on flutter pub run build_runner build
body: <p>I am facing this issue when I run flutter pub run build_runner build. I have tried solutions like flutter clean, flutter pub-cache repair flutter pub get, etc. Dependencies like Mockito, HTTP, and build_runner are added on pubspec and flutter pub get ran successfully as well.</p>
  <pre><code>Failed to precompile build_runner:build_runner:
  ../../snap/flutter/common/flutter/.pub-cache/hosted/pub.dartlang.org/analyzer-1.6.0/lib/src/error/best_practices_verifier.dart:1600:40: Error: Getter not found: 'topLevelVariable'.
        return kinds.contains(TargetKind.topLevelVariable);
                                         ^^^^^^^^^^^^^^^^
  ../../snap/flutter/common/flutter/.pub-cache/hosted/pub.dartlang.org/analyzer-1.6.0/lib/src/error/best_practices_verifier.dart:2024:23: Error: Getter not found: 'topLevelVariable'.
        case TargetKind.topLevelVariable:
                        ^^^^^^^^^^^^^^^^
  ../../snap/flutter/common/flutter/.pub-cache/hosted/pub.dartlang.org/analyzer-1.6.0/lib/src/error/best_practices_verifier.dart:2024:23: Error: Type 'dynamic' of the case expression is not a subtype of type 'TargetKind' of this switch expression.
   - 'TargetKind' is from 'package:meta/meta_meta.dart' ('../../snap/flutter/common/flutter/.pub-cache/hosted/pub.dartlang.org/meta-1.3.0/lib/meta_meta.dart').
        case TargetKind.topLevelVariable:
                        ^
  ../../snap/flutter/common/flutter/.pub-cache/hosted/pub.dartlang.org/analyzer-1.6.0/lib/src/error/best_practices_verifier.dart:2001:13: Context: The switch expression is here.
      switch (this) {
              ^
  pub finished with exit code 1
  </code></pre>