emit a header comment

By tagging every clutz-generated file with a comment, tsickle can
then use this to identify clutz output.

Note that emitComment() output doesn't show up in tests, so this doesn't
affect any tests.  I think it's not very valuable to have this test
baselines include this comment anyway.

Author: evmar

src/main/java/com/google/javascript/clutz/DeclarationGenerator.java

@@ -450,11 +450,15 @@ private String getNamespace(String input) {
   }
 
   String produceDts(Depgraph depgraph) {
+    out = new StringWriter();
+
+    // Note: the specific emit of this header is depended upon by tsickle.
+    emitComment("generated by clutz.");
+
     // Tree sets for consistent order.
     TreeSet<String> provides = new TreeSet<>();
     Set<String> rewrittenProvides = new TreeSet<>();
     Set<String> transitiveProvides = new TreeSet<>();
-    out = new StringWriter();
 
     for (CompilerInput compilerInput : compiler.getInputsById().values()) {
       if (shouldSkipSourceFile(compilerInput.getSourceFile())) {