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

pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart

Index: pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
index 7e8fd9126f660acd977adb6a42eccf29accfacc3..3bf38a3d50aeb30d091fea8021c7085ef731e5e5 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
@@ -2378,6 +2378,17 @@ class KernelVariableDeclaration extends VariableDeclaration
 
   final bool _isLocalFunction;
 
+  /// If this variable declaration represents a formal parameter, indicates
+  /// whether uses of it need to be type-checked due to the covariance of class
+  /// type parameters.
+  ///
+  /// TODO(paulberry): replace this with an enum so that it can represent
+  /// "unsafe" parameters as well.
+  ///
+  /// TODO(paulberry): remove this once there is a corresponding annotation in
+  /// the kernel representation.
+  bool isSemiSafe = false;

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

As for the future direction, I suggest these are represented as annotations
similarly to how we encode native functions.

Unfortunately, we haven't yet implemented metadata on VariableDeclaration, see
issue https://github.com/dart-lang/sdk/issues/28434

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

Let's talk with the kernel folks about this.  My gut feeling is that we
shouldn't use annotations for this, because annotations are meant for
information supplied by the user, whereas this information is supplied by the
front end and should be completely transparent to the user.

But I'm happy to go with whatever representation makes the most sense to the
kernel folks.

+
   KernelVariableDeclaration(String name, this._functionNestingLevel,
       {Expression initializer,
       DartType type,