Bump to Dart SDK `1.21.0-dev.0.0`. (#6372)

Bump to latest Dart SDK dev build (`1.21.0-dev.0.0`).

* updates to analyzer w/ support for bazel workspaces
* removes deprecated analysis `cacheSize` variable access

bin/internal/dart-sdk.version

-1.20.1
+1.21.0-dev.0.0

packages/flutter_tools/lib/src/dart/analysis.dart

@@ -215,7 +215,6 @@ class DriverOptions extends AnalysisOptionsImpl {
 
   DriverOptions() {
     // Set defaults.
-    cacheSize = 512;
     lint = true;
     generateSdkErrors = false;
     trackCacheDependencies = false;