Support the workflow under Windows. google#9

I found and fixed the first file/path issue, while running j2cl native with Windows 10 ... added the following fix ".replace('\\', '/')" here "com.google.j2cl.generator.OutputGeneratorStage#generateOutputs:87"

String typeRelativePath = getRelativePath(type).replace('\\', '/');

Not sure if this fix is well placed, but is corrects the issue (looking up an windows path in a map with unix paths)...

transpiler/java/com/google/j2cl/generator/OutputGeneratorStage.java

@@ -84,7 +84,7 @@ public void generateOutputs(List<CompilationUnit> j2clCompilationUnits) {
             new JavaScriptImplGenerator(problems, declareLegacyNamespace, type);
 
         // If the java type contains any native methods, search for matching native file.
-        String typeRelativePath = getRelativePath(type);
+        String typeRelativePath = getRelativePath(type).replace('\\', '/');
         String typeAbsolutePath =
             FrontendUtils.getJavaPath(getAbsolutePath(j2clCompilationUnit, type));