feat(compiler-cli): add watch mode to ngc (#18818)

With this change ngc now accepts a `-w` or a `--watch`
command-line option that will automatically perform a
recompile whenever any source files change on disk.

PR Close #18818

packages/compiler/src/aot/static_symbol_resolver.ts

@@ -250,6 +250,24 @@ export class StaticSymbolResolver {
     return this.staticSymbolCache.get(declarationFile, name, members);
   }
 
+  /**
+   * hasDecorators checks a file's metadata for the presense of decorators without evalutating the
+   * metada.
+   *
+   * @param filePath the absolute path to examine for decorators.
+   * @returns true if any class in the file has a decorator.
+   */
+  hasDecorators(filePath: string): boolean {
+    const metadata = this.getModuleMetadata(filePath);
+    if (metadata['metadata']) {
+      return Object.keys(metadata['metadata']).some((metadataKey) => {
+        const entry = metadata['metadata'][metadataKey];
+        return entry && entry.__symbolic === 'class' && entry.decorators;
+      });
+    }
+    return false;
+  }
+
   getSymbolsOf(filePath: string): StaticSymbol[] {
     // Note: Some users use libraries that were not compiled with ngc, i.e. they don't
     // have summaries, only .d.ts files. So we always need to check both, the summary