[dart2js] Fix spurious `!` in ConstantMap.

Change-Id: I45ab75005c4f3d658c476886ede3ba37dbb067e9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/204625
Commit-Queue: Mayank Patke <fishythefish@google.com>
Reviewed-by: Stephen Adams <sra@google.com>

https://dart.googlesource.com/sdk/+/c015e02180d0e4231eacc71817f6230182203bc2
diff --git a/commits.json b/commits.json
index 3f3ded4..6c77696 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "external/github.com/flutter/engine":"99b752ed2c8f87bb1d13d89a7cee3fb9caa3b1f2",
   "external/github.com/flutter/flutter":"b9bf5769338d52f33e7d8734c59fb9d7463e6471",
-  "sdk":"11991056b6511ed71aacdfebe72664f5632c5238"
+  "sdk":"c015e02180d0e4231eacc71817f6230182203bc2"
 }
\ No newline at end of file