[CFE] Cache checkInheritanceConflict calculation

When compiling a big internal app with the NNBD SDK there's a big time
regression. This is a first "poor-mans fix" that "fixes" it via caching
which probably is not the best solution, but does the trick.

Without change
==============

Without NNBD platform:

$ out/ReleaseX64/dart pkg/front_end/tool/_fasta/compile.dart --target=dart2js --platform=out/ReleaseX64/dart2js_platform.dill -v bigapp.dart
[...]
0:00:14.238352: Built class hierarchy in 1663ms.
[...]
0:00:55.871356: Wrote component to bigapp.dart.dill in 5690ms.

With NNBD platform:

$ out/ReleaseX64/dart pkg/front_end/tool/_fasta/compile.dart --target=dart2js --platform=out/ReleaseX64NNBD/dart2js_platform.dill -v bigapp.dart
[...]
0:04:41.842548: Built class hierarchy in 268716ms.
[...]
0:05:22.120455: Wrote component to bigapp.dart.dill in 5407ms.

So: Big regression; it takes almost 4.5 minutes to built the class
hierarchy and almost as long extra to compile as a whole.

With change
===========

Without NNBD platform:

$ out/ReleaseX64/dart pkg/front_end/tool/_fasta/compile.dart --target=dart2js --platform=out/ReleaseX64/dart2js_platform.dill -v bigapp.dart
[...]
0:00:15.255893: Built class hierarchy in 2175ms.
[...]
0:00:55.203753: Wrote component to bigapp.dart.dill in 5306ms.

So: Without the NNBD platform there's basically no change.

With NNBD platform:

$ out/ReleaseX64/dart pkg/front_end/tool/_fasta/compile.dart --target=dart2js --platform=out/ReleaseX64NNBD/dart2js_platform.dill -v bigapp.dart
[...]
0:00:15.864901: Built class hierarchy in 2574ms.
[...]
0:00:55.640201: Wrote component to bigapp.dart.dill in 5363ms.

So: With the NNBD platform, the speedup is significant and there's basically no
change between using the NNBD platform or not.
Change-Id: I74993710a34f08f69421b03f2682ff8418af7599
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/141982
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>

https://dart.googlesource.com/sdk/+/47b57e8995478243e99553160e139f51b296b34f
diff --git a/commits.json b/commits.json
index 67ea532..885d28f 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "external/github.com/flutter/engine":"767b48abc91e474ed170b573ee1262ee1088bb0e",
   "external/github.com/flutter/flutter":"da036c521425081b17f1a4a2340086c99716bbb7",
-  "sdk":"278a2db4df1a1fef349b89aeaca367dc7463bab7"
+  "sdk":"47b57e8995478243e99553160e139f51b296b34f"
 }
\ No newline at end of file