test(language-service): Improve integration test (#28168)

The current integration test for language service involves piping the
results of one process to another using Unix pipes.
This makes the test hard to debug, and hard to configure.

This commit refactors the integration test to use regular Jasmine
scaffolding.

More importantly, it tests the way the language service will actually
be installed by end users. Users would not have to add
`@angular/language-service` to the plugins section in tsconfig.json
Instead, all they need to do is install the *extension* from
the VS Code Marketplace and Angular Language Service will be loaded
as a global plugin.

PR Close #28168

integration/language_service_plugin/tsclient.ts

@@ -0,0 +1,70 @@
+import { ChildProcess } from "child_process";
+import { EventEmitter } from "events";
+
+/**
+ * Provides a client for tsserver. Tsserver does not use standard JSON-RPC
+ * protocol thus the need for this custom client.
+ */
+export class Client {
+  private data: Buffer|undefined;
+  private id = 0;
+  private responseEmitter = new EventEmitter();
+
+  constructor(private readonly server: ChildProcess) {}
+
+  listen() {
+    this.server.stdout.on('data', (data: Buffer) => {
+      this.data = this.data ? Buffer.concat([this.data, data]) : data;
+      const CONTENT_LENGTH = 'Content-Length: '
+      const index = this.data.indexOf(CONTENT_LENGTH);
+      if (index < 0) {
+        return;
+      }
+      let start = index + CONTENT_LENGTH.length;
+      let end = this.data.indexOf('\r\n', start);
+      if (end < start) {
+        return;
+      }
+      const contentLengthStr = this.data.slice(start, end).toString();
+      const contentLength = Number(contentLengthStr);
+      if (isNaN(contentLength) || contentLength < 0) {
+        return;
+      }
+      start = end + 4;
+      end = start + contentLength;
+      if (end > this.data.length) {
+        return;
+      }
+      const content = this.data.slice(start, end).toString();
+      this.data = this.data.slice(end);
+      try {
+        const payload = JSON.parse(content);
+        if (payload.type === "event") {
+          return;
+        }
+        this.responseEmitter.emit('response', payload);
+      }
+      catch (error) {
+        this.responseEmitter.emit('error', error);
+      }
+    });
+  }
+
+  async send(type: string, command: string, params: {}) {
+    const request = {
+      seq: this.id++,
+      type,
+      command,
+      arguments: params
+    };
+    this.server.stdin.write(JSON.stringify(request) + '\r\n');
+    return new Promise((resolve, reject) => {
+      this.responseEmitter.once('response', resolve);
+      this.responseEmitter.once('error', reject);
+    });
+  }
+
+  async sendRequest(command: string, params: {}) {
+    return this.send('request', command, params);
+  }
+}