feat(ivy): add getBaseClassIdentifier() to ReflectionHost (#31544)

This method will be useful for writing ngcc `Migrations` that
need to be able to find base classes.

PR Close #31544

packages/compiler-cli/src/ngtsc/reflection/src/host.ts

@@ -505,6 +505,16 @@ export interface ReflectionHost {
    */
   hasBaseClass(clazz: ClassDeclaration): boolean;
 
+  /**
+   * Get an expression representing the base class (if any) of the given `clazz`.
+   *
+   * This expression is most commonly an Identifier, but is possible to inherit from a more dynamic
+   * expression.
+   *
+   * @param clazz the class whose base we want to get.
+   */
+  getBaseClassExpression(clazz: ClassDeclaration): ts.Expression|null;
+
   /**
    * Get the number of generic type parameters of a given class.
    *

packages/compiler-cli/src/ngtsc/reflection/src/typescript.ts

@@ -121,10 +121,28 @@ export class TypeScriptReflectionHost implements ReflectionHost {
   }
 
   hasBaseClass(clazz: ClassDeclaration): boolean {
-    return ts.isClassDeclaration(clazz) && clazz.heritageClauses !== undefined &&
+    return (ts.isClassDeclaration(clazz) || ts.isClassExpression(clazz)) &&
+        clazz.heritageClauses !== undefined &&
         clazz.heritageClauses.some(clause => clause.token === ts.SyntaxKind.ExtendsKeyword);
   }
 
+  getBaseClassExpression(clazz: ClassDeclaration): ts.Expression|null {
+    if (!(ts.isClassDeclaration(clazz) || ts.isClassExpression(clazz)) ||
+        clazz.heritageClauses === undefined) {
+      return null;
+    }
+    const extendsClause =
+        clazz.heritageClauses.find(clause => clause.token === ts.SyntaxKind.ExtendsKeyword);
+    if (extendsClause === undefined) {
+      return null;
+    }
+    const extendsType = extendsClause.types[0];
+    if (extendsType === undefined) {
+      return null;
+    }
+    return extendsType.expression;
+  }
+
   getDeclarationOfIdentifier(id: ts.Identifier): Declaration|null {
     // Resolve the identifier to a Symbol, and return the declaration of that.
     let symbol: ts.Symbol|undefined = this.checker.getSymbolAtLocation(id);