Fix warnings for referenced non-hoisted functions.

Fixes #1034

Also added `expect_warnings` functionality to test framework.

lib/compress.js

@@ -801,7 +801,9 @@ merge(Compressor.prototype, {
     };
 
     function extract_declarations_from_unreachable_code(compressor, stat, target) {
-        compressor.warn("Dropping unreachable code [{file}:{line},{col}]", stat.start);
+        if (!(stat instanceof AST_Defun)) {
+            compressor.warn("Dropping unreachable code [{file}:{line},{col}]", stat.start);
+        }
         stat.walk(new TreeWalker(function(node){
             if (node instanceof AST_Definitions) {
                 compressor.warn("Declarations in unreachable code! [{file}:{line},{col}]", node.start);