Prepare query_benchmark/ for NNBD (#451)

diff --git a/query_benchmark/bin/decode.dart b/query_benchmark/bin/decode.dart
index 83de4b6..bf6e0ff 100644
--- a/query_benchmark/bin/decode.dart
+++ b/query_benchmark/bin/decode.dart
@@ -7,7 +7,8 @@
 import 'package:query_benchmark/readfile.dart';
 
 main() {
-  String path = const String.fromEnvironment('testfile') ?? 'testdata/500.pb';
+  final path =
+      const String.fromEnvironment('testfile', defaultValue: 'testdata/500.pb');
 
   List<int> encoded = readfile(path);
   print(
diff --git a/query_benchmark/bin/decode_json.dart b/query_benchmark/bin/decode_json.dart
index 64d365d..8901b91 100644
--- a/query_benchmark/bin/decode_json.dart
+++ b/query_benchmark/bin/decode_json.dart
@@ -7,7 +7,8 @@
 import 'package:query_benchmark/readfile.dart';
 
 main() {
-  String path = const String.fromEnvironment('testfile') ?? 'testdata/500.pb';
+  final path =
+      const String.fromEnvironment('testfile', defaultValue: 'testdata/500.pb');
 
   List<int> encoded = readfile(path);
   f0.A0 a = f0.A0.fromBuffer(encoded);
diff --git a/query_benchmark/bin/encode.dart b/query_benchmark/bin/encode.dart
index 2fb2d53..93a2619 100644
--- a/query_benchmark/bin/encode.dart
+++ b/query_benchmark/bin/encode.dart
@@ -7,7 +7,9 @@
 import 'package:query_benchmark/readfile.dart';
 
 main() {
-  String path = const String.fromEnvironment('testfile') ?? 'testdata/500.pb';
+  final path =
+      const String.fromEnvironment('testfile', defaultValue: 'testdata/500.pb');
+
   List<int> encoded = readfile(path);
   f0.A0 a = f0.A0.fromBuffer(encoded);
   print(
diff --git a/query_benchmark/bin/encode_json.dart b/query_benchmark/bin/encode_json.dart
index faa95ff..00d5a99 100644
--- a/query_benchmark/bin/encode_json.dart
+++ b/query_benchmark/bin/encode_json.dart
@@ -7,7 +7,8 @@
 import 'package:query_benchmark/readfile.dart';
 
 main() {
-  String path = const String.fromEnvironment('testfile') ?? 'testdata/500.pb';
+  final path =
+      const String.fromEnvironment('testfile', defaultValue: 'testdata/500.pb');
 
   List<int> encoded = readfile(path);
   f0.A0 a = f0.A0.fromBuffer(encoded);
diff --git a/query_benchmark/bin/set_nested_value.dart b/query_benchmark/bin/set_nested_value.dart
index 94698a0..05b26d0 100644
--- a/query_benchmark/bin/set_nested_value.dart
+++ b/query_benchmark/bin/set_nested_value.dart
@@ -9,7 +9,8 @@
 import 'package:query_benchmark/readfile.dart';
 
 main() {
-  String path = const String.fromEnvironment('testfile') ?? 'testdata/500.pb';
+  final path =
+      const String.fromEnvironment('testfile', defaultValue: 'testdata/500.pb');
 
   List<int> encoded = readfile(path);
   f0.A0 a = f0.A0.fromBuffer(encoded)..freeze();
diff --git a/query_benchmark/lib/benchmark.dart b/query_benchmark/lib/benchmark.dart
index 1c35553..2bae31c 100644
--- a/query_benchmark/lib/benchmark.dart
+++ b/query_benchmark/lib/benchmark.dart
@@ -17,6 +17,6 @@
   return s.elapsed ~/ iterations;
 }
 
-String formatReport({String title, Duration duration}) {
+String formatReport({required String title, required Duration duration}) {
   return "RunTimeRaw($title): ${duration.inMicroseconds} us";
 }
diff --git a/query_benchmark/pubspec.yaml b/query_benchmark/pubspec.yaml
index 54a9833..f264e55 100644
--- a/query_benchmark/pubspec.yaml
+++ b/query_benchmark/pubspec.yaml
@@ -6,7 +6,7 @@
 description: Measuring encoding and decoding of a "real-world" protobuf.
 
 environment:
-  sdk: '>=2.3.0 <3.0.0'
+  sdk: '>=2.12.0-0 <3.0.0'
 
 dependencies:
   protobuf: