Start implementing logic for determining when formal parameters need type checks.

pkg/front_end/lib/src/fasta/source/source_loader.dart

Index: pkg/front_end/lib/src/fasta/source/source_loader.dart
diff --git a/pkg/front_end/lib/src/fasta/source/source_loader.dart b/pkg/front_end/lib/src/fasta/source/source_loader.dart
index 7a0500fb694aa33b9f7e7edc325cf260925cfa3d..fcf8f3b2c6a0ce1c7c9f07e270677e6df79d925c 100644
--- a/pkg/front_end/lib/src/fasta/source/source_loader.dart
+++ b/pkg/front_end/lib/src/fasta/source/source_loader.dart
@@ -8,7 +8,10 @@ import 'dart:async' show Future;
 
 import 'dart:typed_data' show Uint8List;
 
-import 'package:kernel/ast.dart' show Arguments, Expression, Program;
+import 'package:front_end/src/fasta/kernel/kernel_library_builder.dart'
+    show KernelLibraryBuilder;
+
+import 'package:kernel/ast.dart' show Arguments, Class, Expression, Program;
 
 import 'package:kernel/class_hierarchy.dart' show ClassHierarchy;
 
@@ -496,6 +499,25 @@ class SourceLoader<L> extends Loader<L> {
     ticker.logMs("Performed initializer inference");
   }
 
+  /// Annotates method formals that require runtime checks to restore soundness
+  /// as a result of the fact that Dart 2.0 treats all class type parameters as
+  /// covariant.
+  void computeFormalSafety() {
+    if (target.strongMode) {
+      var unorderedClasses = <Class>[];
+      builders.forEach((Uri uri, LibraryBuilder library) {

[ahe, 2017/08/23 09:55:58]

Can you pass in this list either from collectAllClasses or
collectAllSourceClasses in kernel_target?

[Paul Berry, 2017/08/23 17:09:10]

Done.

+        if (library is KernelLibraryBuilder) {
+          var builtLibrary = library.target;
+          unorderedClasses.addAll(builtLibrary.classes);
+        }
+      });
+      for (var cls in hierarchy.getOrderedClasses(unorderedClasses)) {
+        typeInferenceEngine.computeFormalSafety(cls);
+      }
+      ticker.logMs("Computed formal checks");
+    }
+  }
+
   List<Uri> getDependencies() => sourceBytes.keys.toList();
 
   Expression instantiateInvocation(Expression receiver, String name,