Remove enableAssertInitializer from analysis options as it's not needed in Dart 2 (#14042)

analysis_options.yaml

diff --git a/analysis_options.yaml b/analysis_options.yaml
index 01199cb..dce1cb1 100644
--- a/analysis_options.yaml
+++ b/analysis_options.yaml
@@ -26,7 +26,6 @@
   language:
     enableStrictCallChecks: true
     enableSuperMixins: true
-    enableAssertInitializer: true
   strong-mode:
     implicit-dynamic: false
   errors:

packages/flutter/lib/analysis_options_user.yaml

diff --git a/packages/flutter/lib/analysis_options_user.yaml b/packages/flutter/lib/analysis_options_user.yaml
index 53f1563..41cb657 100644
--- a/packages/flutter/lib/analysis_options_user.yaml
+++ b/packages/flutter/lib/analysis_options_user.yaml
@@ -23,7 +23,6 @@
   language:
     enableStrictCallChecks: true
     enableSuperMixins: true
-    enableAssertInitializer: true
   strong-mode: true
   errors:
     # treat missing required parameters as a warning (not a hint)