Update tree walking to not update a list while iterating over it.

tools/checkdeps/java_checker.py

Index: tools/checkdeps/java_checker.py
diff --git a/tools/checkdeps/java_checker.py b/tools/checkdeps/java_checker.py
index 36e46230f4cd6ee47160fd27cd8555eb31898e98..8b33199f9e663fea0c8da4a3db8c684d58ae1d35 100644
--- a/tools/checkdeps/java_checker.py
+++ b/tools/checkdeps/java_checker.py
@@ -39,25 +39,28 @@ class JavaChecker(object):
     self._classmap = {}
     self._PrescanFiles()
 
+  def _IgnoreDir(self, d):
+    # Skip hidden directories.
+    if d.startswith('.'):
+      return True
+    # Skip the "out" directory, as dealing with generated files is awkward.
+    # We don't want paths like "out/Release/lib.java" in our DEPS files.
+    # TODO(husky): We need some way of determining the "real" path to
+    # a generated file -- i.e., where it would be in source control if
+    # it weren't generated.
+    if d == 'out':
+      return True
+    # Skip third-party directories.
+    if d in ('third_party', 'ThirdParty'):
+      return True
+    return False
+
   def _PrescanFiles(self):
     for root, dirs, files in os.walk(self._base_directory):
       # Skip unwanted subdirectories. TODO(husky): it would be better to do
       # this via the skip_child_includes flag in DEPS files. Maybe hoist this
       # prescan logic into checkdeps.py itself?
-      for d in dirs:
-        # Skip hidden directories.
-        if d.startswith('.'):
-          dirs.remove(d)
-        # Skip the "out" directory, as dealing with generated files is awkward.
-        # We don't want paths like "out/Release/lib.java" in our DEPS files.
-        # TODO(husky): We need some way of determining the "real" path to
-        # a generated file -- i.e., where it would be in source control if
-        # it weren't generated.
-        if d == 'out':
-          dirs.remove(d)
-        # Skip third-party directories.
-        if d in ('third_party', 'ThirdParty'):
-          dirs.remove(d)
+      dirs[:] = [d for d in dirs if not self._IgnoreDir(d)]
       for f in files:
         if f.endswith('.java'):
           self._PrescanFile(os.path.join(root, f))