[analyzer] `Bad state` in `_assertLegacyType` with a String type

[asashour]

On trying to add some null safety expectations to a test (as part of #51144), there is a state error:

Bad state: Expected all legacy types, but got String (InterfaceTypeImpl)

I wonder what _assertLegacyType should do when encountered with a String type, because changing the last condition to be

    var nullabilitySuffix = type.nullabilitySuffix;
    if (nullabilitySuffix == NullabilitySuffix.star ||
        nullabilitySuffix == NullabilitySuffix.none) {
      return;
    }

would fail LegacyTypeAsserterTest

Stacktrace

Bad state: Expected all legacy types, but got String (InterfaceTypeImpl)
#0      LegacyTypeAsserter._assertLegacyType (package:analyzer/src/dart/resolver/legacy_type_asserter.dart:163:5)
#1      LegacyTypeAsserter.visitExpression (package:analyzer/src/dart/resolver/legacy_type_asserter.dart:75:5)
#2      GeneralizingAstVisitor.visitLiteral (package:analyzer/dart/ast/visitor.dart:478:36)
#3      GeneralizingAstVisitor.visitStringLiteral (package:analyzer/dart/ast/visitor.dart:669:48)
#4      GeneralizingAstVisitor.visitSingleStringLiteral (package:analyzer/dart/ast/visitor.dart:658:7)
#5      GeneralizingAstVisitor.visitSimpleStringLiteral (package:analyzer/dart/ast/visitor.dart:655:7)
#6      SimpleStringLiteralImpl.accept (package:analyzer/src/dart/ast/ast.dart:12073:50)
#7      PartOfDirectiveImpl.visitChildren (package:analyzer/src/dart/ast/ast.dart:10313:11)
#8      GeneralizingAstVisitor.visitNode (package:analyzer/dart/ast/visitor.dart:526:10)
#9      GeneralizingAstVisitor.visitAnnotatedNode (package:analyzer/dart/ast/visitor.dart:135:48)
#10     GeneralizingAstVisitor.visitDirective (package:analyzer/dart/ast/visitor.dart:271:40)
#11     GeneralizingAstVisitor.visitPartOfDirective (package:analyzer/dart/ast/visitor.dart:560:52)
#12     PartOfDirectiveImpl.accept (package:analyzer/src/dart/ast/ast.dart:10307:50)
#13     NodeListImpl.accept (package:analyzer/src/dart/ast/ast.dart:9589:20)
#14     CompilationUnitImpl.visitChildren (package:analyzer/src/dart/ast/ast.dart:2572:19)
#15     GeneralizingAstVisitor.visitNode (package:analyzer/dart/ast/visitor.dart:526:10)
#16     GeneralizingAstVisitor.visitCompilationUnit (package:analyzer/dart/ast/visitor.dart:217:52)
#17     LegacyTypeAsserter.visitCompilationUnit (package:analyzer/src/dart/resolver/legacy_type_asserter.dart:63:13)
#18     LegacyTypeAsserter.assertLegacyTypes (package:analyzer/src/dart/resolver/legacy_type_asserter.dart:169:26)
#19     Iterable.every (dart:core/iterable.dart:403:16)
#20     LibraryAnalyzer._computeDiagnostics (package:analyzer/src/dart/analysis/library_analyzer.dart:304:25)
#21     LibraryAnalyzer.analyze (package:analyzer/src/dart/analysis/library_analyzer.dart:92:5)
#22     AnalysisDriver._computeAnalysisResult.<anonymous closure> (package:analyzer/src/dart/analysis/driver.dart:1347:11)
<asynchronous suspension>
#23     PerformanceLog.runAsync (package:analyzer/src/dart/analysis/performance_logger.dart:50:14)
<asynchronous suspension>
#24     AnalysisDriver.performWork (package:analyzer/src/dart/analysis/driver.dart:1055:24)
<asynchronous suspension>
#25     AnalysisDriverScheduler._run (package:analyzer/src/dart/analysis/driver.dart:2139:7)
<asynchronous suspension>

[bwilkerson]

@scheglov