diff --git a/build/build-cdt-lib.js b/build/build-cdt-lib.js
index 6802709c7563..2e84aec7cb8a 100644
--- a/build/build-cdt-lib.js
+++ b/build/build-cdt-lib.js
@@ -26,8 +26,7 @@ execFileSync('node_modules/.bin/tsc', [
 console.log('making modifications ...');
 
 for (const file of glob.sync(`${outDir}/**/*.js`)) {
-  const code = fs.readFileSync(file, 'utf-8')
-    .replace('exports["default"]', 'exports.default');
+  const code = fs.readFileSync(file, 'utf-8');
 
   let lines = code.match(/^.*(\r?\n|$)/mg) || [];
   const cutoffIndex = lines.findIndex(line => line.includes('Legacy exported object'));
@@ -43,6 +42,8 @@ for (const file of glob.sync(`${outDir}/**/*.js`)) {
     if (file.endsWith('ParsedURL.js')) {
       newLine = newLine.replace(/Common.ParsedURL/g, 'ParsedURL');
     }
+    newLine = newLine.replace('exports["default"]', 'exports.default');
+
     if (newLine !== line) {
       // eslint-disable-next-line no-console
       console.log(`${file}:${i}: ${line.trim()}`);
diff --git a/lighthouse-core/lib/cdt/SDK.js b/lighthouse-core/lib/cdt/SDK.js
index 63bc63173d0a..f3607f31ba0f 100644
--- a/lighthouse-core/lib/cdt/SDK.js
+++ b/lighthouse-core/lib/cdt/SDK.js
@@ -4,7 +4,6 @@
  * Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.
  */
 'use strict';
-// @ts-nocheck
 
 const SDK = {
   ...require('./generated/sdk/SourceMap.js'),
@@ -18,6 +17,9 @@ SDK.TextSourceMap.prototype.mappings = function() {
 };
 
 /**
+ * Copied from CDT utilities.js. This is the only method needed from
+ * that file.
+ *
  * Return index of the leftmost element that is greater
  * than the specimen object. If there's no such element (i.e. all
  * elements are smaller or equal to the specimen) returns right bound.
@@ -34,6 +36,7 @@ SDK.TextSourceMap.prototype.mappings = function() {
  * @template T,S
  */
 function upperBound(object, comparator, left, right) {
+  // @ts-ignore
   function defaultComparator(a, b) {
     return a < b ? -1 : (a > b ? 1 : 0);
   }
@@ -74,12 +77,14 @@ SDK.TextSourceMap.prototype.findExactEntry = function(line, column) {
 // Add `lastColumnNumber` to mappings.
 SDK.TextSourceMap.prototype.computeLastGeneratedColumns = function() {
   const mappings = this.mappings();
+  // @ts-ignore: `lastColumnNumber` is not on types yet.
   if (mappings.length && typeof mappings[0].lastColumnNumber !== 'undefined') return;
 
   for (let i = 0; i < mappings.length - 1; i++) {
     const mapping = mappings[i];
     const nextMapping = mappings[i + 1];
     if (mapping.lineNumber === nextMapping.lineNumber) {
+      // @ts-ignore: `lastColumnNumber` is not on types yet.
       mapping.lastColumnNumber = nextMapping.columnNumber;
     }
   }