fix(ngcc): handle missing sources when flattening source-maps (#35718)

If a package has a source-map but it does not provide
the actual content of the sources, then the source-map
flattening was crashing.

Now we ignore such mappings that have no source
since we are not able to compute the merged
mapping if there is no source file.

Fixes #35709

PR Close #35718

packages/compiler-cli/ngcc/src/sourcemaps/source_file.ts

@@ -290,11 +290,16 @@ export function parseMappings(
     const generatedLineMappings = rawMappings[generatedLine];
     for (const rawMapping of generatedLineMappings) {
       if (rawMapping.length >= 4) {
+        const originalSource = sources[rawMapping[1] !];
+        if (originalSource === null || originalSource === undefined) {
+          // the original source is missing so ignore this mapping
+          continue;
+        }
         const generatedColumn = rawMapping[0];
         const name = rawMapping.length === 5 ? rawMap.names[rawMapping[4]] : undefined;
         const mapping: Mapping = {
           generatedSegment: {line: generatedLine, column: generatedColumn},
-          originalSource: sources[rawMapping[1] !] !,
+          originalSource,
           originalSegment: {line: rawMapping[2] !, column: rawMapping[3] !}, name
         };
         mappings.push(mapping);

packages/compiler-cli/ngcc/test/sourcemaps/source_file_spec.ts

@@ -174,6 +174,28 @@ runInEachFileSystem(() => {
             },
           ]);
         });
+
+        it('should ignore mappings to missing source files', () => {
+          const bSourceMap: RawSourceMap = {
+            mappings: encode([[[1, 0, 0, 0], [4, 0, 0, 3], [4, 0, 0, 6], [5, 0, 0, 7]]]),
+            names: [],
+            sources: ['c.js'],
+            version: 3
+          };
+          const bSource = new SourceFile(_('/foo/src/b.js'), 'abcdef', bSourceMap, false, [null]);
+          const aSourceMap: RawSourceMap = {
+            mappings: encode([[[0, 0, 0, 0], [2, 0, 0, 3], [4, 0, 0, 2], [5, 0, 0, 5]]]),
+            names: [],
+            sources: ['b.js'],
+            version: 3
+          };
+          const aSource =
+              new SourceFile(_('/foo/src/a.js'), 'abdecf', aSourceMap, false, [bSource]);
+
+          // These flattened mappings are just the mappings from a to b.
+          // (The mappings to c are dropped since there is no source file to map to.)
+          expect(aSource.flattenedMappings).toEqual(parseMappings(aSourceMap, [bSource]));
+        });
       });
 
       describe('renderFlattenedSourceMap()', () => {