Allow dart pub deps --json to fail without causing an explicit crash. (#166778)

Closes https://github.com/flutter/flutter/issues/166648.

I think a better fix is to stop using `dart pub deps --json` and use
@sigurdm's new `.dart_tool/` vendored solution, but that can be a
follow-up change.
https://dart.googlesource.com/external/github.com/flutter/flutter/+/54b3a97f5399cba702a1fcdc510988a4aac5a381
diff --git a/DEPS b/DEPS
index 654eb5a..b5cfc27 100644
--- a/DEPS
+++ b/DEPS
@@ -240,7 +240,7 @@
 deps = {
   'flutter/engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + '72264d1af2532950073fa47e7517dc391a6d53b7',
 
-  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '4673e20c727fffdaac9d5a660997e2fd0b796659',
+  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '54b3a97f5399cba702a1fcdc510988a4aac5a381',
 
   'flutter/engine/src/flutter/third_party/depot_tools':
   Var('chromium_git') + '/chromium/tools/depot_tools.git' + '@' + '580b4ff3f5cd0dcaa2eacda28cefe0f45320e8f7',
diff --git a/commits.json b/commits.json
index 96f2a4e..5f8a34d 100644
--- a/commits.json
+++ b/commits.json
@@ -1,4 +1,4 @@
 {
-  "flutter":"4673e20c727fffdaac9d5a660997e2fd0b796659",
+  "flutter":"54b3a97f5399cba702a1fcdc510988a4aac5a381",
   "flutter/engine/src/flutter/third_party/dart":"72264d1af2532950073fa47e7517dc391a6d53b7"
 }
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