diff --git a/scripts/gulpfiles/build_tasks.js b/scripts/gulpfiles/build_tasks.js
index 810bb425c..e6a395c40 100644
--- a/scripts/gulpfiles/build_tasks.js
+++ b/scripts/gulpfiles/build_tasks.js
@@ -630,6 +630,15 @@ function buildCompiled() {
     // option to Closure Compiler; instead feed them as input via gulp.src.
   };
 
+  // Symlink source dirs from build dir so that sourcemaps work in
+  // compiled-mode testing.
+  for (const src of ['core', 'blocks', 'generators']) {
+    const target = `${BUILD_DIR}/${src}`
+    if (!fs.existsSync(target)) {
+      fs.symlinkSync(`../${src}`, target);
+    }
+  }
+
   // Fire up compilation pipline.
   return gulp.src(chunkOptions.js, {base: './'})
       .pipe(stripApacheLicense())
diff --git a/tsconfig.json b/tsconfig.json
index 3b1971dc5..9dbdfcfab 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -11,8 +11,9 @@
     // normally they are ignored as source files
     "allowJs": true,
 
-    // Generate d.ts files
+    // Generate d.ts files and sourcemaps.
     "declaration": true,
+    "sourceMap": true,
 
     "module": "ES2015",
     "moduleResolution": "node",