Treat type inference context `dynamic` as equivalent to an empty context.

pkg/front_end/testcases/inference/downward_inference_fixes_no_upwards_errors.dart.strong.expect

Index: pkg/front_end/testcases/inference/downward_inference_fixes_no_upwards_errors.dart.strong.expect
diff --git a/pkg/front_end/testcases/inference/downward_inference_fixes_no_upwards_errors.dart.strong.expect b/pkg/front_end/testcases/inference/downward_inference_fixes_no_upwards_errors.dart.strong.expect
new file mode 100644
index 0000000000000000000000000000000000000000..924976eff6c96a9cb7bc2c9cee8d183c4a0237c8
--- /dev/null
+++ b/pkg/front_end/testcases/inference/downward_inference_fixes_no_upwards_errors.dart.strong.expect
@@ -0,0 +1,14 @@
+library test;
+import self as self;
+import "dart:core" as core;
+import "dart:math" as math;
+
+static method f() → dynamic {
+  core::num x;
+  dynamic y;
+  core::num a = math::max<core::num>(x, y);
+  core::Object b = math::max<core::num>(x, y);
+  dynamic c = math::max<dynamic>(x, y);
+  dynamic d = math::max<dynamic>(x, y);
+}
+static method main() → dynamic {}