harden the implementation of Program.getSourceFile to handle all relative file names correctly

Author: vladima

src/compiler/program.ts

@@ -568,7 +568,9 @@ namespace ts {
         }
 
         function getSourceFile(fileName: string) {
-            return filesByName.get(fileName);
+            // first try to use file name as is to find file
+            // then try to convert relative file name to absolute and use it to retrieve source file
+            return filesByName.get(fileName) || filesByName.get(getNormalizedAbsolutePath(fileName, host.getCurrentDirectory()));
         }
 
         function getDiagnosticsHelper(

tests/cases/unittests/moduleResolution.ts

@@ -208,6 +208,10 @@ export = C;
            assert.equal(syntacticDiagnostics.length, 0, `expect no syntactic diagnostics, got: ${JSON.stringify(syntacticDiagnostics.map(diagnosticToString))}`);
            const semanticDiagnostics = program.getSemanticDiagnostics();
            assert.equal(semanticDiagnostics.length, 0, `expect no semantic diagnostics, got: ${JSON.stringify(semanticDiagnostics.map(diagnosticToString))}`);
+
+           // try to get file using a relative name
+           const fileC = program.getSourceFile("../../../c/third/class_c.ts");
+           assert.isTrue(fileC !== undefined, `expected to get file by relative name, got ${fileC}`);
        });
 
         function diagnosticToString(diagnostic: Diagnostic) {