fix(compiler): add an empty content for source file of non mapped code. (#15246)

Before this when using ngc, tools tried to load `ng://…<component>.ts`
if `…<component>.ts` was the source file of a template.

PR Close #15246

packages/compiler/src/output/abstract_emitter.ts

@@ -98,7 +98,10 @@ export class EmitterVisitorContext {
     let firstOffsetMapped = false;
     const mapFirstOffsetIfNeeded = () => {
       if (!firstOffsetMapped) {
-        map.addSource(sourceFilePath).addMapping(0, sourceFilePath, 0, 0);
+        // Add a single space so that tools won't try to load the file from disk.
+        // Note: We are using virtual urls like `ng:///`, so we have to
+        // provide a content here.
+        map.addSource(sourceFilePath, ' ').addMapping(0, sourceFilePath, 0, 0);
         firstOffsetMapped = true;
       }
     };

packages/compiler/test/aot/compiler_spec.ts

@@ -188,7 +188,7 @@ describe('compiler (unbundled Angular)', () => {
              // for the mapping to the original source file we don't store the source code
              // as we want to keep whatever TypeScript / ... produced for them.
              const sourceIndex = sourceMap.sources.indexOf(ngComponentPath);
-             expect(sourceMap.sourcesContent[sourceIndex]).toBe(null);
+             expect(sourceMap.sourcesContent[sourceIndex]).toBe(' ');
            });
          }));
 

packages/compiler/test/output/js_emitter_node_only_spec.ts

@@ -57,7 +57,7 @@ export function main() {
         const sm = emitSourceMap(someVar.toStmt(), [], '/* MyPreamble \n */');
 
         expect(sm.sources).toEqual([someSourceFilePath, 'in.js']);
-        expect(sm.sourcesContent).toEqual([null, ';;;var']);
+        expect(sm.sourcesContent).toEqual([' ', ';;;var']);
         expect(originalPositionFor(sm, {line: 3, column: 0}))
             .toEqual({line: 1, column: 3, source: 'in.js'});
       });

packages/compiler/test/output/ts_emitter_node_only_spec.ts

@@ -62,7 +62,7 @@ export function main() {
         const sm = emitSourceMap(someVar.toStmt(), [], '/* MyPreamble \n */');
 
         expect(sm.sources).toEqual([someSourceFilePath, 'in.js']);
-        expect(sm.sourcesContent).toEqual([null, ';;;var']);
+        expect(sm.sourcesContent).toEqual([' ', ';;;var']);
         expect(originalPositionFor(sm, {line: 3, column: 0}))
             .toEqual({line: 1, column: 3, source: 'in.js'});
       });

packages/core/test/linker/source_map_integration_node_only_spec.ts

@@ -128,7 +128,7 @@ export function main() {
            expect(sourceMap.sources).toEqual([
              'ng:///DynamicTestModule/MyComp.ngfactory.js', ngUrl
            ]);
-           expect(sourceMap.sourcesContent).toEqual([null, template]);
+           expect(sourceMap.sourcesContent).toEqual([' ', template]);
          }));