Fix nullable Future.value and Completer.complete cases

In these two cases, a nullable value is passed where a runtime error
will occur if the value is null. Make runtime check explicit.

Cleanup for #53253

Change-Id: Ia14bd345cbe20c50c75bdd2069f44449157bcc36
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/321423
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Paul Berry <paulberry@google.com>

pkg/analyzer/lib/src/dart/analysis/driver.dart

@@ -1134,7 +1134,7 @@ class AnalysisDriver implements AnalysisDriverGeneric {
     if (_unitElementRequestedFiles.isNotEmpty) {
       String path = _unitElementRequestedFiles.keys.first;
       var completers = _unitElementRequestedFiles.remove(path)!;
-      final result = await _computeUnitElement(path);
+      final result = (await _computeUnitElement(path))!;
       for (var completer in completers) {
         completer.complete(result);
       }

pkg/front_end/lib/src/fasta/kernel/macro/macro.dart

@@ -1385,7 +1385,7 @@ class _DefinitionPhaseIntrospector extends _DeclarationPhaseIntrospector
   @override
   Future<macro.TypeAnnotation> inferType(
           macro.OmittedTypeAnnotation omittedType) =>
-      new Future.value(macroApplications._inferOmittedType(omittedType));
+      new Future.value(macroApplications._inferOmittedType(omittedType)!);
 
   @override
   Future<List<macro.Declaration>> topLevelDeclarationsOf(