Add type inference for `this` expressions.

pkg/front_end/testcases/inference/this_reference.dart.direct.expect

Index: pkg/front_end/testcases/inference/this_reference.dart.direct.expect
diff --git a/pkg/front_end/testcases/inference/this_reference.dart.direct.expect b/pkg/front_end/testcases/inference/this_reference.dart.direct.expect
new file mode 100644
index 0000000000000000000000000000000000000000..ea33b64a9a2e2908eb30e3c799912c848d5941ab
--- /dev/null
+++ b/pkg/front_end/testcases/inference/this_reference.dart.direct.expect
@@ -0,0 +1,13 @@
+library test;
+import self as self;
+import "dart:core" as core;
+
+class C<T extends core::Object> extends core::Object {
+  constructor •() → void
+    : super core::Object::•()
+    ;
+  method f() → void {
+    dynamic x = this;
+  }
+}
+static method main() → dynamic {}