fix(language-service): remove asserts for non-null expressions (#16422)

Reworked some of the code so asserts are no longer necessary.
Added additional and potentially redundant checks
Added checks where the null checker found real problems.

PR Close #16422

packages/language-service/src/reflector_host.ts

@@ -22,18 +22,24 @@ class ReflectorModuleModuleResolutionHost implements ts.ModuleResolutionHost {
     if (snapshot) {
       return snapshot.getText(0, snapshot.getLength());
     }
+
+    // Typescript readFile() declaration should be `readFile(fileName: string): string | undefined
     return undefined !;
   }
 
   directoryExists: (directoryName: string) => boolean;
 }
 
+// This reflector host's purpose is to first set verboseInvalidExpressions to true so the
+// reflector will collect errors instead of throwing, and second to all deferring the creation
+// of the program until it is actually needed.
 export class ReflectorHost extends CompilerHost {
   constructor(
       private getProgram: () => ts.Program, serviceHost: ts.LanguageServiceHost,
       options: AngularCompilerOptions) {
     super(
-        null !, options,
+        // The ancestor value for program is overridden below so passing null here is safe.
+        /* program */ null !, options,
         new ModuleResolutionHostAdapter(new ReflectorModuleModuleResolutionHost(serviceHost)),
         {verboseInvalidExpression: true});
   }