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

pkg/front_end/testcases/runtime_checks/covariant_generic_parameter.dart.strong.expect

+library test;
+import self as self;
+import "dart:core" as core;
+
+class C<T extends core::Object> extends core::Object {
+  default constructor •() → void
+    : super core::Object::•()
+    ;
+  method f(self::C::T x) → void {}
+}
+static method g1(self::C<core::num> c) → void {
+  c.{self::C::f}(1.5);
+}
+static method g2(self::C<core::int> c) → void {
+  c.{self::C::f}(1);
+}
+static method g3(self::C<core::num> c) → void {
+  c.{self::C::f}(null);
+}
+static method main() → dynamic {}