Roll src/third_party/dart 3173e59..a8b6e18 (16 commits) (flutter#8883)

a8b6e18 Resolve x ?? y as LUB(X!, Y) instead of LUB(X, Y).
78e50a5 Revert "Implement the type Never"
986c079 Build local elements in constructor initializers.
00c6c53 Include 'exists' into unlinked signature.
32f146a Fix tests that are probably opinions how something should be resolved.
d2d8e8a Prefer hasLibraryDirective over hasPartOfDirective.
c331ce6 Update extract implicitly typed instance property tests.
dd6da83 [CFE] Avoid cloning bodies of lazy if elements.
3e95afb [CFE] Refactor const collection construction into builders.
a83fbc5 remove unused enable lazyAssignmentOperators flag
ff75c98 Remove two generated files unintentionally included in 100244.
9e90424 Decouple startup emitter from compiler/backend
3d7405f Separate CodegenImpl from JavaScriptBackend
1c1af28 Introduce Codegen interface to decouple JavaScriptBackend from the codegen phase
f1fcd89 [SDK] Fix missing argument in tools/bots/dart_sdk.py
4934f76 [vm] protobuf aware treeshaker, Support freezing messages

DEPS

@@ -31,7 +31,7 @@ vars = {
   # 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': '3173e594baf4894b1edfd7bebae2c7721729d38d',
+  'dart_revision': 'a8b6e18b6224e7d65caedd80478861f52568de67',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py
@@ -74,7 +74,7 @@ vars = {
   'dart_path_tag': '1.6.2',
   'dart_pedantic_tag': 'v1.5.0',
   'dart_pool_tag': '1.3.6',
-  'dart_protobuf_rev': '0c77167b16d00b561a6055bfe26690af7f26ae88',
+  'dart_protobuf_rev': '7d34c9e4e552a4f66acce32e4344ae27756a1949',
   'dart_pub_rev': '8c363fe26f059c3063f1129adbb3c4e22a8ce954',
   'dart_pub_semver_tag': '1.4.2',
   'dart_quiver-dart_tag': '2.0.0+1',

ci/licenses_golden/licenses_third_party

@@ -1,4 +1,4 @@
-Signature: 43c2ac333cd581a5a7c93f09c2014b2d
+Signature: 3af2390b8fdd028b5143cb8fc947ab94
 
 UNUSED LICENSES: