smith: Use strict-casts instead of no-implicit-casts

These modes are almost identical; one difference which is highlighted
below is that a `dynamic` value used in a spread is considered to be
an implicit cast from dynamic to Iterable.

Bug: https://github.com/dart-lang/sdk/issues/47902
Change-Id: Iabb6881c72e29349c36ce99e29853a8315b6f4dd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/225044
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>

pkg/smith/analysis_options.yaml

@@ -1,3 +1,3 @@
 analyzer:
-  strong-mode:
-    implicit-casts: false
+  language:
+    strict-casts: true

pkg/smith/lib/builder.dart

@@ -73,11 +73,12 @@ class Step {
         throw FormatException("Step tests multiple configurations: $arguments");
       }
     }
+    var environment = map["environment"] as Map<Object?, Object?>?;
     return Step(
         map["name"] as String,
         script,
         arguments,
-        <String, String>{...?map["environment"]},
+        {...?environment?.cast<String, String>()},
         map["fileset"] as String?,
         map["shards"] as int?,
         isTestRunner,
@@ -192,14 +193,17 @@ List<Builder> parseBuilders(
   var names = <String>{};
   for (var builderConfiguration in builderConfigurations) {
     var meta = builderConfiguration["meta"] as Map? ?? <String, String>{};
-    var builderNames = <String>[...?builderConfiguration["builders"]];
-    var steps = <Map>[...?builderConfiguration["steps"]];
-    for (var builderName in builderNames) {
-      if (!names.add(builderName)) {
-        throw FormatException('Duplicate builder name: "$builderName"');
+    var builderNames = builderConfiguration["builders"] as List<Object?>?;
+    if (builderNames != null) {
+      var steps = ((builderConfiguration["steps"] ?? []) as List<Object?>)
+          .cast<Map<Object?, Object?>>();
+      for (var builderName in builderNames.cast<String>()) {
+        if (!names.add(builderName)) {
+          throw FormatException('Duplicate builder name: "$builderName"');
+        }
+        builders.add(Builder.parse(builderName, steps, configurations,
+            meta["description"] as String?));
       }
-      builders.add(Builder.parse(
-          builderName, steps, configurations, meta["description"] as String?));
     }
   }
   return builders;

pkg/smith/lib/test_matrix.dart

@@ -52,9 +52,13 @@ class TestMatrix {
       }
     });
 
-    var builderConfigurations = <Map>[...?json["builder_configurations"]];
+    var builderConfigurations = [
+      ...?(json["builder_configurations"] as List<Object?>?)
+          ?.cast<Map<Object?, Object?>>()
+    ];
     var builders = parseBuilders(builderConfigurations, configurations);
-    var branches = <String>[...?json["branches"]];
+    var branchesValue = json["branches"] as List?;
+    var branches = [...?branchesValue?.cast<String>()];
 
     // Check that each configuration is tested on at most one builder.
     var testedOn = <Configuration, Builder>{};