Bump Dart SDK to latest (`1.21.0-dev.2.0`). (#6541)

bin/internal/dart-sdk.version

-1.21.0-dev.0.0
+1.21.0-dev.2.0

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

@@ -121,8 +121,10 @@ class AnalysisDriver {
     }
 
     if (options.packageRootPath != null) {
-      ContextBuilder builder = new ContextBuilder(resourceProvider, null, null);
-      builder.defaultPackagesDirectoryPath = options.packageRootPath;
+      ContextBuilderOptions builderOptions = new ContextBuilderOptions();
+      builderOptions.defaultPackagesDirectoryPath = options.packageRootPath;
+      ContextBuilder builder = new ContextBuilder(resourceProvider, null, null, 
+          options: builderOptions);
       PackageMapUriResolver packageUriResolver = new PackageMapUriResolver(resourceProvider,
           builder.convertPackagesToMap(builder.createPackageMap('')));