fix(language-service): Diagnostic span should point to class name (#34932)

Right now, if an Angular diagnostic is generated for a TypeScript node,
the span points to the decorator Identifier, i.e. the Identifier node
like `@NgModule`, `@Component`, etc.
This is weird. It should point to the class name instead.
Note, we do not have a more fine-grained breakdown of the span when
diagnostics are emitted, this work remains to be done.

PR Close #34932

packages/language-service/test/diagnostics_spec.ts

@@ -362,9 +362,7 @@ describe('diagnostics', () => {
         .toBe(
             `Component 'MyComponent' is not included in a module and will not be available inside a template. Consider adding it to a NgModule declaration.`);
     const content = mockHost.readFile(fileName) !;
-    const keyword = '@Component';
-    expect(start).toBe(content.lastIndexOf(keyword) + 1);  // exclude leading '@'
-    expect(length).toBe(keyword.length - 1);               // exclude leading '@'
+    expect(content.substring(start !, start ! + length !)).toBe('MyComponent');
   });
 
 
@@ -604,9 +602,7 @@ describe('diagnostics', () => {
         .toBe(
             'Invalid providers for "AppComponent in /app/app.component.ts" - only instances of Provider and Type are allowed, got: [?null?]');
     // TODO: Looks like this is the wrong span. Should point to 'null' instead.
-    const keyword = '@Component';
-    expect(start).toBe(content.lastIndexOf(keyword) + 1);  // exclude leading '@'
-    expect(length).toBe(keyword.length - 1);               // exclude leading '@
+    expect(content.substring(start !, start ! + length !)).toBe('AppComponent');
   });
 
   // Issue #15768
@@ -775,8 +771,7 @@ describe('diagnostics', () => {
       const {file, messageText, start, length} = diags[0];
       expect(file !.fileName).toBe(APP_COMPONENT);
       expect(messageText).toBe(`Component 'AppComponent' must have a template or templateUrl`);
-      expect(start).toBe(content.indexOf(`@Component`) + 1);
-      expect(length).toBe('Component'.length);
+      expect(content.substring(start !, start ! + length !)).toBe('AppComponent');
     });
 
     it('should report diagnostic for both template and templateUrl', () => {
@@ -795,8 +790,7 @@ describe('diagnostics', () => {
       expect(file !.fileName).toBe(APP_COMPONENT);
       expect(messageText)
           .toBe(`Component 'AppComponent' must not have both template and templateUrl`);
-      expect(start).toBe(content.indexOf(`@Component`) + 1);
-      expect(length).toBe('Component'.length);
+      expect(content.substring(start !, start ! + length !)).toBe('AppComponent');
     });
 
     it('should report errors for invalid styleUrls', () => {