Stop accepting null as a context in _fe_analyzer_shared.

Now that the analyzer no longer makes a distinction between `null` and
`_` as contexts (https://dart-review.googlesource.com/c/sdk/+/357521),
no longer uses `null` to represent contexts
(https://dart-review.googlesource.com/c/sdk/+/357522), and no longer
passes `null` as a context to the shared analysis logic
(https://dart-review.googlesource.com/c/sdk/+/358662), we can safely
update the shared analysis logic so that it no longer accepts `null`
as a context.

Change-Id: I0512d59405255ef45497885e7668ebc3b12abf58
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/358800
Reviewed-by: Chloe Stefantsova <[email protected]>
Commit-Queue: Paul Berry <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>

pkg/_fe_analyzer_shared/lib/src/type_inference/type_analyzer.dart

@@ -529,9 +529,9 @@ mixin TypeAnalyzer<
   /// [schema] is the type schema which should be used for type inference.
   ///
   /// Stack effect: pushes (Expression).
-  Type analyzeExpression(Expression node, TypeSchema? schema) {
+  Type analyzeExpression(Expression node, TypeSchema schema) {
     // Stack: ()
-    if (schema == null || operations.typeSchemaIsDynamic(schema)) {
+    if (operations.typeSchemaIsDynamic(schema)) {
       schema = operations.unknownType;
     }
     ExpressionTypeAnalysisResult<Type> result =