Don't generate import statements for unused imports.

R=r
http://codereview.appspot.com/979046
diff --git a/compiler/testdata/test.proto b/compiler/testdata/test.proto
index ef91b03..2f64132 100644
--- a/compiler/testdata/test.proto
+++ b/compiler/testdata/test.proto
@@ -32,6 +32,7 @@
 package my.test;  // dotted package name
 
 import "imp.proto";
+import "multi1.proto";  // unused import
 
 enum HatType {
   // deliberately skipping 0