Roll to branched dart sdk with two more hotfixes for flutter 1.5.4. (#8790)

* Roll to branched dart sdk with two more hotfixes for fluter 1.5.4.

Cherry-picks for https://github.com/flutter/flutter/issues/29007 and VERSION file bump.

* Update license hash

DEPS

diff --git a/DEPS b/DEPS
index afa4a87..60b822b 100644
--- a/DEPS
+++ b/DEPS
@@ -31,7 +31,7 @@
   # Dart is: https://github.com/dart-lang/sdk/blob/master/DEPS.
   # You can use //tools/dart/create_updated_flutter_deps.py to produce
   # updated revision list of existing dependencies.
-  'dart_revision': '9818857822aac8f84f36d84e8eea6c554dad5615',
+  'dart_revision': 'a1668566e563aef64025d0af88a099cbbe847b7e',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py

ci/licenses_golden/licenses_third_party

diff --git a/ci/licenses_golden/licenses_third_party b/ci/licenses_golden/licenses_third_party
index ebbcabf..1297199 100644
--- a/ci/licenses_golden/licenses_third_party
+++ b/ci/licenses_golden/licenses_third_party
@@ -1,4 +1,4 @@
-Signature: cb9e296575fc050579bacb3a1c06c9b5
+Signature: 8559a97367500dd3793df745b4b2a42f
 
 UNUSED LICENSES: