Implementing fix and test for case when we have multiple sources but mapping refers to the single source only.

Author: dmurat

lib/source-map-generator.js

@@ -72,15 +72,13 @@ SourceMapGenerator.fromSourceMap =
       generator.addMapping(newMapping);
     });
     aSourceMapConsumer.sources.forEach(function (sourceFile) {
-      if (aSourceMapConsumer._mappings === '') {
-        var sourceRelative = sourceFile;
-        if (sourceRoot !== null) {
-          sourceRelative = util.relative(sourceRoot, sourceFile);
-        }
+      var sourceRelative = sourceFile;
+      if (sourceRoot !== null) {
+        sourceRelative = util.relative(sourceRoot, sourceFile);
+      }
 
-        if (!generator._sources.has(sourceRelative)) {
-          generator._sources.add(sourceRelative);
-        }
+      if (!generator._sources.has(sourceRelative)) {
+        generator._sources.add(sourceRelative);
       }
 
       var content = aSourceMapConsumer.sourceContentFor(sourceFile);

test/test-source-map-generator.js

@@ -270,6 +270,12 @@ exports['test .fromSourceMap with empty mappings and relative sources'] = functi
   util.assertEqualMaps(assert, map.toJSON(), util.testMapEmptyMappingsRelativeSources_generatedExpected);
 };
 
+exports['test .fromSourceMap with multiple sources where mappings refers only to single source'] = function (assert) {
+    var map = SourceMapGenerator.fromSourceMap(
+        new SourceMapConsumer(util.testMapMultiSourcesMappingRefersSingleSourceOnly));
+    util.assertEqualMaps(assert, map.toJSON(), util.testMapMultiSourcesMappingRefersSingleSourceOnly);
+};
+
 exports['test applySourceMap'] = function (assert) {
   var node = new SourceNode(null, null, null, [
     new SourceNode(2, 0, 'fileX', 'lineX2\n'),

test/util.js

@@ -95,6 +95,14 @@ exports.testMapEmptyMappingsRelativeSources_generatedExpected = {
   sourceRoot: '/the/root',
   mappings: ''
 };
+exports.testMapMultiSourcesMappingRefersSingleSourceOnly = {
+    version: 3,
+    file: 'min.js',
+    names: ['bar', 'baz'],
+    sources: ['one.js', 'withoutMappings.js'],
+    sourceRoot: '',
+    mappings: 'CAAC,IAAI,IAAM,SAAUA,GAClB,OAAOC,IAAID'
+};
 // This mapping is identical to above, but uses the indexed format instead.
 exports.indexedTestMap = {
   version: 3,