Stricter analyzer tests (#143)

- Rename analysis_options.yaml
- Treat some hints as errors
- Remove an unused import

Author: natebosch

.analysis_options renamed to analysis_options.yaml

@@ -1,5 +1,10 @@
 analyzer:
  strong-mode: true
+ errors:
+   unused_import: error
+   unused_local_variable: error
+   invalid_assignment: error
+   dead_code: erro
 linter:
   rules:
      # Errors

lib/src/builder.dart

@@ -4,7 +4,6 @@
 import 'dart:async';
 
 import 'package:analyzer/dart/element/element.dart';
-import 'package:analyzer/exception/exception.dart';
 import 'package:build/build.dart';
 import 'package:dart_style/src/dart_formatter.dart';