Merge pull request #493 from RReverser/patch-1

Fix for importing modules from definitions

src/services/services.ts

@@ -1411,10 +1411,7 @@ module ts {
             return {
                 getSourceFile: (filename, languageVersion) => {
                     var sourceFile = getSourceFile(filename);
-
-                    Debug.assert(!!sourceFile, "sourceFile can not be undefined");
-
-                    return sourceFile;
+                    return sourceFile && sourceFile.getSourceFile();
                 },
                 getCancellationToken: () => cancellationToken,
                 getCanonicalFileName: (filename) => useCaseSensitivefilenames ? filename : filename.toLowerCase(),