Roll Dart to df5036eb6e738c723339ed74c1e8ca93bea2570d.

[rmacnak-google]

df5036e [vm] Require only an isolate group, not an isolate, in Dart_Delete[Weak]PersistentHandle.
f67cc65 Update tests and error code for void with type arguments.
09e27af [nnbd_migration] Improve dartdev migrate command for the "apply" button.
d83efc3 [nnbd_migration] Apply the changes to disk on apply-migration request
9eb655e [nnbd_migration] Preserve "migration applied" state across reload.
28a607c [nnbd_migration] Send request to server from apply migration button.
99d02a1 [nnbd_migration] "Apply migration" button ux/styles
bb40d68 quote fix
fc1d2a8 [dartfix] remove the 'dartfix upgrade sdk' command (prefer 'dartdev migrate')
ed1091a Add two fixes for avoid_returning_null_for_future
33d6e68 [dart2js] Fix narrowType to handle NeverType in assert.
27c50ae Implement factor() operation from flow analysis.
81bc629 Use packageDefaultFeatureSet when the language version is the same as the current version.
1f92f1d Issue 40865. Fix for parsing documentation comment of an annotatied EnumConstantDeclaration.
3c2a6c2 [dart2js] Erase '*' in runtime type objects.
a0c3b4f [dart2js] Fix some types in NNBD SDK.
fb547fc Log data to debug crash in declaredParameterElement().
159fcee [dart2js] Fix type inference with Future.then
54903e5 [vm/ffi] Rename members in NativeApi to not contain nativeApi
a8c7757 [CFE] Don't hard-code uncertain version numbers in messages or tests
4e2a1a1 [CFE/kernel] Generate dart files with versions in them from tools/experimental_features.yaml
f828f00 [parser] Error recovery of type parameters on void
067c1e0 [nnbd/dartdevc] enforce cfe checking on sdk
eca9aab [cfe] Enable NNBD support in VM for NNBD unit tests
70b4ffa [kernel] Delete limited_ast_to_binary.dart
12df258 [kernel] Small cleanup of binary printer
0738b67 [cfe] Allow nullable right-hand side in typedefs
800008b [infra] Run more migrated tests on the analyzer-nnbd builder
38cc00c [infra] Balance the number shards on the AOT builder with co19_2 tests
23106da [VM/Testing] - Added co19_2 tests to vm-precompiled-linux-release-x64 builds
3d5fb65 [cfe] Handle implicit extension method call in argument replacement
1b41780 [kernel] Investigative inspection into BinaryPrinter

[cbracken]

This caused significant breakage and necessitated a co-ordinated Dart SDK roll internally. While this isn't a breaking change for Flutter users (and the number of these changes has historically actually pretty low), it would be great to see if there are ways we could land changes like this in some incremental fashion in order to keep rolls running smoothly.

FWIW I haven't actually looked at the change in detail -- so no idea if there was a way we could have staged this incrementally without a tonne of effort.