diff --git a/packages/compiler-cli/src/ngtsc/testing/src/utils.ts b/packages/compiler-cli/src/ngtsc/testing/src/utils.ts
index 13fe87a560..fe72f5e67f 100644
--- a/packages/compiler-cli/src/ngtsc/testing/src/utils.ts
+++ b/packages/compiler-cli/src/ngtsc/testing/src/utils.ts
@@ -138,3 +138,12 @@ function bindingNameEquals(node: ts.BindingName, name: string): boolean {
   }
   return false;
 }
+
+export function getSourceCodeForDiagnostic(diag: ts.Diagnostic): string {
+  if (diag.file === undefined || diag.start === undefined || diag.length === undefined) {
+    throw new Error(
+        `Unable to get source code for diagnostic. Provided diagnostic instance doesn't contain "file", "start" and/or "length" properties.`);
+  }
+  const text = diag.file.text;
+  return text.substr(diag.start, diag.length);
+}
diff --git a/packages/compiler-cli/test/ngtsc/template_typecheck_spec.ts b/packages/compiler-cli/test/ngtsc/template_typecheck_spec.ts
index 1cc1497288..a6c5d44954 100644
--- a/packages/compiler-cli/test/ngtsc/template_typecheck_spec.ts
+++ b/packages/compiler-cli/test/ngtsc/template_typecheck_spec.ts
@@ -11,7 +11,7 @@ import * as ts from 'typescript';
 import {ErrorCode, ngErrorCode} from '../../src/ngtsc/diagnostics';
 import {absoluteFrom as _, getFileSystem, getSourceFileOrError} from '../../src/ngtsc/file_system';
 import {runInEachFileSystem} from '../../src/ngtsc/file_system/testing';
-import {expectCompleteReuse, loadStandardTestFiles} from '../../src/ngtsc/testing';
+import {expectCompleteReuse, getSourceCodeForDiagnostic, loadStandardTestFiles} from '../../src/ngtsc/testing';
 
 import {NgtscTestEnvironment} from './env';
 
@@ -2506,8 +2506,3 @@ export declare class AnimationEvent {
     });
   });
 });
-
-function getSourceCodeForDiagnostic(diag: ts.Diagnostic): string {
-  const text = diag.file!.text;
-  return text.substr(diag.start!, diag.length!);
-}