Update left top rule order in constraint solving (#1200)

Fix issue from #1035 . Update the algorithm for constraint solving to prefer decomposing the `dynamic` or `void` types on the left into `Object?` when matching against `Q?` for some `Q`.

Author: leafpetersen

resources/type-system/inference.md

@@ -6,6 +6,9 @@ Status: Draft
 
 ## CHANGELOG
 
+2019.09.01
+  - Fix incorrect placement of left top rule in constraint solving.
+
 2019.09.01
   - Add left top rule to constraint solving.
   - Specify inference constraint solving.
@@ -774,7 +777,9 @@ clauses need be tried.
 - If `P` is a legacy type `P0*` then the match holds under constraint set `C`:
   - Only if `P0` is a subtype match for `Q` under constraint set `C`.
 - If `Q` is a legacy type `Q0*` then the match holds under constraint set `C`:
-  - Only if `P` is a subtype match for `Q?` under constraint set `C`.
+  - If `P` is `dynamic` or `void` and `P` is a subtype match for `Q0` under
+    constraint set `C`.
+  - Or if `P` is a subtype match for `Q0?` under constraint set `C`.
 - If `Q` is `FutureOr<Q0>` the match holds under constraint set `C`:
   - If `P` is `FutureOr<P0>` and `P0` is a subtype match for `Q0` under
     constraint set `C`.
@@ -786,22 +791,21 @@ clauses need be tried.
 - If `Q` is `Q0?` the match holds under constraint set `C`:
   - If `P` is `P0?` and `P0` is a subtype match for `Q0` under
     constraint set `C`.
+  - Or if `P` is `dynamic` or `void` and `Object` is a subtype match for `Q0`
+    under constraint set `C`.
   - Or if `P` is a subtype match for `Q0` under **non-empty** constraint set
-    `C`
-  - Or if `P` is a subtype match for `Null` under constraint set `C`
+    `C`.
+  - Or if `P` is a subtype match for `Null` under constraint set `C`.
   - Or if `P` is a subtype match for `Q0` under **empty** constraint set
-    `C`
+    `C`.
 - If `P` is `FutureOr<P0>` the match holds under constraint set `C1 + C2`:
   - If `Future<P0>` is a subtype match for `Q` under constraint set `C1`
   - And if `P0` is a subtype match for `Q` under constraint set `C2`
 - If `P` is `P0?` the match holds under constraint set `C1 + C2`:
   - If `P0` is a subtype match for `Q` under constraint set `C1`
   - And if `Null` is a subtype match for `Q` under constraint set `C2`
-
 - If `Q` is `dynamic`, `Object?`, or `void` then the match holds under no
   constraints.
-- If `P` is `dynamic` or `void` then the match holds under constraint set `C` if
-  `Object?` is a subtype match for `Q` under constraint set `C`.
 - If `P` is `Never` then the match holds under no constraints.
 - If `Q` is `Object`, then the match holds under no constraints:
   - Only if `P` is non-nullable.