feat(compiler-cli): expose ngtsc as a TscPlugin (#28435)

This lets us run ngtsc under the tsc_wrapped custom compiler (Used in Bazel)
It also allows others to simply wire ngtsc into an existing typescript compilation binary

PR Close #28435

packages/compiler-cli/src/ngtsc/tsc_plugin.ts

@@ -0,0 +1,78 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {PluginCompilerHost, TscPlugin} from '@bazel/typescript/tsc_wrapped/plugin_api';
+import * as ts from 'typescript';
+
+import {SyntheticFilesCompilerHost} from './synthetic_files_compiler_host';
+
+// Copied from tsc_wrapped/plugin_api.ts to avoid a runtime dependency on that package
+function createProxy<T>(delegate: T): T {
+  const proxy = Object.create(null);
+  for (const k of Object.keys(delegate)) {
+    proxy[k] = function() { return (delegate as any)[k].apply(delegate, arguments); };
+  }
+  return proxy;
+}
+
+export class NgTscPlugin implements TscPlugin {
+  constructor(private angularCompilerOptions: unknown) {}
+
+  wrap(program: ts.Program, config: {}, host: ts.CompilerHost) {
+    const proxy = createProxy(program);
+    proxy.getSemanticDiagnostics = (sourceFile: ts.SourceFile) => {
+      const result: ts.Diagnostic[] = [...program.getSemanticDiagnostics(sourceFile)];
+
+      // For demo purposes, trigger a diagnostic when the sourcefile has a magic string
+      if (sourceFile.text.indexOf('diag') >= 0) {
+        const fake: ts.Diagnostic = {
+          file: sourceFile,
+          start: 0,
+          length: 3,
+          messageText: 'Example Angular Compiler Diagnostic',
+          category: ts.DiagnosticCategory.Error,
+          code: 12345,
+          // source is the name of the plugin.
+          source: 'Angular',
+        };
+        result.push(fake);
+      }
+      return result;
+    };
+    return proxy;
+  }
+
+  createTransformers(host: PluginCompilerHost) {
+    const afterDeclarations: Array<ts.TransformerFactory<ts.SourceFile|ts.Bundle>> =
+        [(context: ts.TransformationContext) => (sf: ts.SourceFile | ts.Bundle) => {
+          const visitor = (node: ts.Node): ts.Node => {
+            if (node.kind === ts.SyntaxKind.ClassDeclaration) {
+              const clz = node as ts.ClassDeclaration;
+              // For demo purposes, transform the class name in the .d.ts output
+              return ts.updateClassDeclaration(
+                  clz, clz.decorators, node.modifiers, ts.createIdentifier('NEWNAME'),
+                  clz.typeParameters, clz.heritageClauses, clz.members);
+            }
+            return ts.visitEachChild(node, visitor, context);
+          };
+          return visitor(sf) as ts.SourceFile;
+        }];
+    return {afterDeclarations};
+  }
+
+  wrapHost(inputFiles: string[], compilerHost: ts.CompilerHost) {
+    return new SyntheticFilesCompilerHost(inputFiles, compilerHost, this.generatedFiles);
+  }
+
+  generatedFiles(rootFiles: string[]) {
+    return {
+      'file-1.ts': (host: ts.CompilerHost) =>
+                       ts.createSourceFile('file-1.ts', 'contents', ts.ScriptTarget.ES5),
+    };
+  }
+}